I'm sure that the Line 6 guys will give you a definitive answer on exactly where their XDR955's dual-diversity frequencies "sit" with respect to the four frequency diversity RF1 scheme you're currently employing in your five XD-V70's.
Until they do, one immediate check you can do would be to first have the XD-V70 receivers powered without any of their transmitters operating and then fire up the XDR955's transmitter.
Red lighting of more than one or two segments of any of your receivers' "RF" LED ladders signals possible interference to the individual channel currently set at a given receiver.
If those red bars appear and disappear in time with switching the XDR955 on and off, then it is indeed most likely what's giving you grief and its re-tuning may be all that's needed. If it's not the culprit, then look for some other local source of interference in the 2.4GHz ISM band, probably WiFi or Bluetooth that's quite close by.
Whatever it turns out to be, ...in the event that re-tuning the XDR955 hasn't eliminated the problem and/or clearance by more distant relocation of other interference source/s isn't possible, re-tune the affected XD-V "chain" (or "chains") to the least red-LEDded channel/s available.
To answer your next question, I've found it really great to have added a single standalone V75 "chain" to my eight V70's (Click on my pink avatar pic for details of my mobile rig.) Aside from working really well together, the V75 grants V70 gear a "future proof" update path as well as more frequency agility and extra features, including improved channel scanning diagnostics.
XD-V beltpacks and handhelds are fully interoperable and interchangeable once a V75 has been employed to re-flash the V70 gear to the latest firmware via Line 6 Monkey.
The only "caveat" is that XD-V75's are only specified to be able to "daisy chain" four receivers from a single pair of antennae, (albeit without needing the "last receiver" termination BNC's required by the older V70's) whereas the V70's can run a daisy chain of up to six.
I guess this would require you to ultimately rig for two pairs of antennae, each pair potentially being able to run four receivers.
(I'd be really tempted to see what signal strengths are like when trying the trusty XDR955 as the last in a daisy chain of four, yielding a much neater "two by four" rig. Why not "suck it and see"? ...There's not a lot to lose...)
As for running two XD-V transmitters on the same channel, it's something I've actually done deliberately (but wouldn't necessarily recommend) to pre-fit a backstage (hence further away) performer's wig mic and beltpack ...while that performer's identically-kitted "channel mate" was actually on stage.
This requires an "ironclad" discipline in coordination. As soon as the onstage performer exits, that transmitter is switched off. The disappearance of that signal forces its receiver to "seek", wherupon it finds and performs a new "handshake" with the next performer's "already on" beltpack. This happens in roughly one second.
As you'll appreciate, very little pre-listen time's available to confirm the "second" beltpack, but it can be a distinct advantage to have like-sounding sources sharing a signal path.
Each transmitter "owns" its channel until switching it off "forces" renewed "seeking". In common with other interference, the closer an identically tuned secondary transmitter comes to the receiving point, the greater the likelihood of "drop-outs" and range limitation affecting the primary source, hence the requirement for discipline.
This presents no problem at all for guitarists, for whom this technique (along with the long battery life of TBP12's) makes the "pre-hanging" of a bunch of beltpacks on shoulder straps an absolute joy ...resulting in the effortless and seamless swapping between different instruments that wirelessly feed just one input to a stomp-box, amp and/or mixer rig.
Unlike analog systems, XD-V's give no "splats" or "fizzes" when switching, re-tuning or performing new "handshakes".
We have had a few occasions where there has been a digital interference coming through them, but we're not sure if it's due to using the XDR955 at the same time as the V70, distance maybe. Would this cause any problems, should we avoid using any specific channels on the 995 or V70?
Not certain exactly what you mean. No interference can cause any audio artifacts in th radio section. If you are hearing something it is because it is being inputted into the system. It is possible on 955 to pickup stray RF interference in the mic capsule itself, so if you have a cell phone very close to the mic for example you could be generating noise into the mic itself. The XD-V's have additional filtering to prevent this. Otherwise interference to any of these systems can shorten the range ... but it does not create audio artifacts, so something else is happening.
We're also looking to purchase 2 more units, I understand that the V70 is no longer available, is there any problem with using the V70 at the same time as the V75. Is there a trade in program to swap out V70's for V75's?
No problem at all. You must however make certain that all the systems are on RF1 or RF 2 mode ... you cannot mix modes. As far as swapping units, that would be at the dealer level.
We've also thought about purchasing body packs to be used with the V70's can the packs be used with the V75 or are they incompatible?
Yes ... V75's can transmit to V70 receivers and vise-versa
What happens if a bodypack and handheld transmitter are on at the same time on the same channel?
If there is some distance between the two transmitters it is likely that the receiver will lock to the closer unit and disregard the further unit. If they are right next to each other it will be a mess.
That all seems to make sense, I'm guessing that the interference was caused by a cell phone and the transmitter. Knowing that the v70's wouldn't cause that kind of issue makes sense then.
Thanks for the information about being able to use V70's and V75's together and specifically that they are software version upgradeable!
I'm not worried about integrating multiple sets of these together.
I guess my only question would be We'd like to use P180's to hopefully move our mics from Hi power to Low Power in order to save battery life. Since we plan to add 2 or 3 V75 units how could we connect all 8 units (5-V70, 3-V75) together with one set of P180's?
I understand that Line 6 are shortly to release a 1 rack unit module that will enable the RF distribution from the single pair of paddles that you have in mind, Andrew...
It will, apparently, facilitate and neaten the supply of DC power to multiple receivers as well.
Until it comes along, it's strictly a maximum daisy chain of four receivers per pair of antennae when V75's are involved, with up to a maximum of six receivers able to be daisy chained in an "all V70" rig.
You say that your receivers are about 100 feet from their transmitters. The most important thing to consider in regard to RF propagation (or any radiation, such as light levels from a lamp or sound levels from a source to a mic, or from loudspeakers to ears) is the "inverse square law".
This means that halving the distance from source to "target" (in this case, transmitters to receiving antennae) results in four times (two squared) the level being received. A quarter the distance, ...sixteen times the level, and so on.
I regularly use my four 50ft LMR195 cables https://www.globalfulfillment.net/gfsnet/(S(u2akra45154po145g2facm3w))/line6/10Expand.aspx?ProductCode=98-033-0018 to do exactly that with the two pairs of P180's that feed my two racks of four XD-V receivers, ...so my transmitters do mostly run at their lower power "Battery Save" setting, just as you're hoping to do.
You'll note that even though my eight V70's could run to a daisy chain of six, I've chosen to divide them into two kits of four in the interests of both RF integrity and operational convenience.
Accordingly, you should be able to arrive at a rig that'll do the same, bearing in mind that the XD-V system's stability is such that immediate front panel access is very rarely required after initial set-up.
This stability often allows the placement of receivers (as well as their antennae) "up close and personal" in wingspace, the consequently staggering "inverse square" gains of received RF levels then yielding amazingly robust reliability.
Backstage audio staff then reach and give such rare "front panel" attention as might be necessary exactly as they would when attending to an individual performer.
Mostly, though, (being a "one man band") I find it's more practical to both reduce distance and simultaneously "back reject" undesirable RF through the judicious location of just my cabled directional antennae.