Hi

Whether the amp models you hear in Line 6 gear are accurate or not is a very subjective thing

It's almost certain that many of the original supposedly identical amps sounded different to each other in minor ways anyway - a bit like when someone claims to sell an accurate modern day replica of a Fender Strat 1959 bridge pickup as in those days pickup bobbins were wound until the bobbin was full with wire - neither the wire gauge nor the number of turns could be guaranteed to be exactly the same every time, so whilst a pickup may well be accurate when compared to the unique original sample pickup used to copy from it will probably sound a bit different to other original Strat pickups produced in 1959.  I don't know exactly how Line 6 achieve their amp models, but I believe they do have a number of original amps which are analysed electronically, digitally and by simply listening to them side by side with the modelled variants.  I think also that amp model settings will be agreed on somewhat by a committee of people who work for Line 6, freelance players and technicians.  On that basis, what you are going to get with any Line 6 modelling amp is a pretty darned close representation of the original amp(s) used to create the models you hear.

Mostly, I think the Line 6 models we get are as accurate as they can ever hope to be, but I don't actually think it's possible to be 100% acccurate as there is no such thing as a standard identically performing amp in the first place due to component tolerances and wear on them.  At the end of the day, the models offered to us are only ever going to be someone else's opinion of what sounds right.  But that said, the models presented are certainly going to be in the right ball-park, so I would expect that what you can hear is going to be at least a 98% accurate representation of what the original modelled amp would also sound like.  Of course to be 100% sure, you would need to find an example of the original amp, set it up in terms of drive and tone the same way as the modelled version and play your guitar through it to compare.  If you're hearing the same kind of overtones in more than just the Spider IV, the chances are that what you are hearing is pretty accurate for the amp that Line 6 actually used when creating the model.

I don't know what guitar you are using, but you've probably heard of so-called 'wolf' tones which are odd harmonics caused by having the pickup pole pieces set too close to the strings.  It may be that you need to adjust the pickup on your guitar so that the pole piece associated with the G string is moved further away.   Some amps enhance certain frequencies more than others - and some famous players have used original AC30 amps for this very reason - notably players like Brian May and Rory Gallagher and others have favoured Marshall for the same reason but different results.

I believe you have used your own guitar with all the tests you tried out in store on the SV etc...?  Can you try a different guitar other than yours with the amp models that are displaying these characteristics you're hearing?  I'm just thinking that it might be something related to your particular guitar such as pickup position, type or string type.  It may even be something to do with the bridge saddle on your specific guitar like a bit of a wire edge or burr.  Obviously you know your own guitar and it probably doesn't display any of the problems you're hearing on any normal amp, but trying another guitar would rule out the problem from being anything to do with your guitar - only a suggestion.

I don't have experience with the original Marshall Plexi amps either, but I have used two or three AC-30's in my time, and you should be aware that not all AC-30's were based on the same design, so I'm not sure which AC-30 model type Line 6 have based their AC-30 model on - I think it might be documented somewhere.

Nick

I posted the same question about the Class A Red model on the Spider IV 75 three months ago:

http://line6.com/community/thread/19556

Do a search for ghost notes on Vox AC 30s, it's a characteristic that has been accurately modelled by Line 6 in their amp models.

Since I also own the new Spider Valve MKII I can tell you it is that way on that amp too. I really like the new Class A models!