Check out @yek 's Guide to the Fractal Audio Amp models. There's a wealth of information about how to dial in different sounds on the USA Clean, USA Lead, USA Rhythm, USA IIC+ and USA IIC++ models.To my ears, the MkV sounds a fair amount different than the 2 and 4 (fixed gain stage, minor eq differences, etc). But I'll take the mkV off my list haha.
I am currently the first owner of the latest GT 100 FV off of Graydon’s bench,only had it a few weeks now, and I have yet to hear it through a proper cabinet, I’ve been using it with my UAD OX since the 100 W Max of my FAS LB 2 may not be safe to run on the B channel of this GT 100 which I was told is close to 200W cranked.Guytron GT-100 or GT-100 F/V
(Skip to 17m27s below)
You know an amp is loved when the leftover tape glue has built up so much it's now the new "tape" for marking amp settings LOL
Jon Herington on the Guytron amp:
I now (2018) have two brand-new GT 100 FV amps. The original GT 100 I had was pretty beaten up after 13 years on the road. I really loved the original amp without the FV controls, but I do use the FV controls on my amps. I like to keep things simple, so in a way I think I would prefer the amp without the FV controls, but I do like the sound of the amp with focus set on the second position and voicing set on the fourth position. That's how I usually set it. I don't think you'll find the amp feels all that much different with the FV controls, though it does give you more flexibility and control, and it certainly does change the response of the amp. If you're anything like I am, you'll like both amps equally well. My recommendation is to listen to an FV if you get a chance, but if the price is a good one for the GT 100, I don't imagine you would ever regret making the purchase, so you might want to grab it even if you don't get to compare the amps.
It's a pretty complicated design, but it's amazingly simple to use and always sounds great, no matter what the volume. I love the channel switching, the effects loops, and the master volume. It's always been my favorite amp because it has the right combination of features, tone, and simplicity.