if you are using a JTV it's really no question, HD500 all the way. You don't need the DT with your current stated situation. The HD500 however is needed for connection, updating and using Workbench HD. You can't do any of that with the Amplifi. The options of getting a good pair of active studio monitors for use will suit you well and you could even take them to a jam session if needed in a pinch.
+1 to that! The HD500+JTV offers so much that it's hard to even sum up. The manner in which you can then amplify the sound expands greatly - the AMPLiFi offers literally *zero* output options outside the use of the speaker itself. If you plug in the 1/8" headphone out jack on the AMPLiFi, it cuts off the speaker. It's really not an equal comparison, the HD500 gear has so much more in terms of 1/4" outputs, XLR outputs, USB to PC recording function, L6Link, an FX loop...
The AMPLiFi is a funky gadget, and while it's fun and looks exciting, it lacks some overall useable control that I value greatly. Wwhereas with the JTV+HD500x, you can do some really, really cool deep explorations in sound, you'll have a whole palette of HD amp models, a host of M-series FX, and a looper, and a tuner, and a volume pedal, and a wah. The VDI connection both powers the JTV, and allows you some next-gen type interfacing - you can program the HD to control parameters on the JTV, like the tuning, the pickup selection, the guitar model - you can even split the variax models and the variax mags into two signals on the HD500 and route them through two different amp models, on two signal paths, out to two different speakers. You can also use the tone and volume knobs on the JTV to control effects parameters on the HD500 - like the number of delay repeats, or the amp model gain, or the reverb mix - any of the FX parameters can be defined as controllable both by those two knobs on the JTV, but also the onboard expression pedal of the HD500 - which can be toggled with the toe switch between EXP-1 and EXP-2. Traditional setup makes those wah and volume pedal. But as radatats mentioned in other posts, you can set those to control *several* different things. You can use those pedals to fade between entire effects and amp bank settings on your A and B channel. You can do ALOT with the JTV+HD500, that you simply cannot do with AMPLIfi.
The Gearbox connectivity was *just* released for the JTV / HD series, so now you can plug your JTV into your HD500, and use it to power the guitar while editing it with Workbench HD. And, you get a decent ASIO soundcard solution with the HD500 USB output if you want to record yourself. None of that is offered on AMPLiFi.
In terms of speakers, you have several options. The two main Line6 offerings - that will interface with the HD500 using L6Link are the DT25 and 50 series amps, and the StageSource speakers. I like them both, each has it's own merits. I've been actually considering merging them into my guitar rig - I already use a DT25 combo, was thinking about sending the XLR DI out to my L2t, sit it on the floor in "tilt back" mode, and place it at the base of the amp stand my DT sits on. Just one of many, many configurations that can be achieved.
I also use a pair of M-Audio powered studio monitors for when I don't want to drag out the whole rig, or if I am adding HD500 direct to some existing recordings. Headphones are always a good option when it's late at night, or my wife is trying to get to sleep early. The HD500 also gives you multiple "output" modes, which can be used to adjust the components of the output. The two main versions are "with microphone and room modeling" and "without". The main distinction for me, is the difference between listening to your amp being mic'd, in another room, and you hear that signal in the control room - or the main mix of the house sound system, vs. being next to your amp, with no mic and room modeling - just amp and cabinet. The DT offers a cabinet emulation XLR output, which essentially creates the "mic and room" mix for sending out to the front of house mix, or to the recording console, or to a FRFR speaker system. Options are GOOD!
I won't try to downplay the Bluetooth function of the AMPLIfI nor will I discount the iPad function of the gear; it's a great idea, but it's not that which limits the gear so much, it's that you cannot reach over to the amp, and adjust any particular setting easily. It's that they went with their older amp modeling tech to offer the "cloud" based tone control. It's that they sacrificed alot to offer some cool new techy stuff. I prefer analog outputs galore and adjust on the fly control, over Bluetooth and iPad control. If you already had a stellar killer rig, and needed a simple, all-in-on rig, the AMPLiFi is probably perfect. But for you, with your JTV - you should definitely take a much closer look at the extra bells and whistles the Dream Rig components offer.
With some practice on the HD500, you will quickly realise how easy it is to build an entire patch from scratch, swapping amp models, assigning FS control to certain FX or parameters - and when you use the HD edit to build and edit patches, it's so easy and fun it's ridiculous. But that's the power to me - if I am at rehearsal and need to come up with a patch or edit on the fly, I can do it quickly and easily with just the HD500 as the interface. No internet required, no iPad...
And yeah, get ready to spend some $$! But you've already got the JTV, so that's a big chunk of the expense. With $500 for the HD500 and some makeshift speaker ensembles, you can do a lot to advance your tone quest. When you are ready to make it a big louder, or find yourself jamming with others, drummers perhaps - then you can look at the DT if you want the tube amp tone - and the Bogner switching is pretty solid. Or if you prefer the full range flat response, then take a look at the L2 and L3 speakers, and compare them to what else is out there in the world of that kind of gear - a keyboard amp for example is a great way to amplify the HD500, can usually be run in stereo, and most often will have appropriate DI / XLR outputs.
So in fact, you are comparing spending $500 on the HD500x vs around $500 on AMPLIfI - and that battle definitely goes to the HD for the win. They really did a bang up awesome job with that piece of gear, and the AMPLiFi just can't hang in that realm.