In my opinion...
The first thing you should do is make the three EQs adjustable in a more straightfoward way. Leave in the HP/LP stuff because it's very cool and will play nicely with the new thing, but as part of GUTS I think you ought to allow the Bass and Treble EQ be switchable between Shelving with a definable center and Q, and Parametric within a defined bandwidth (say 90hz to 300hz for the bass, and from 3500hz to 7000hz for the treble); the midrange should be Parametric as well. This will allow users to tune the EQ of the amp much more specifically, and aid GREATLY in emulating existing amps as tone stack behavior can be more realistically achieved this way.
I would consider adding another parametric strip to your Cabinet Sim, as well; right now it allows you to shape the tone, but not to really sculpt it, but adding another band would probably make that possible. Even though your non-IR cabinet sim is one of the best non-IR, relatively non-complex cabinet sims I've ever used, if I may be frank I still think that it is the weakest element in getting a realistic tone simply because though the filtration does cut down the frequencies not present in a real guitar amp and cabinet system, the subtle interferences, resonances, and modulation of a real cabinet are poorly captured in any non-convolution based system that isn't extremely sophisticated. Your approach is very effective for getting a "DI box" tone that you can tune any way you like, but it needs to become more sophisticated if users are to be able to use it as the primary cabinet simulation for distribution of GUTS files (and I think that should be your goal, as it removes any problems associated with asking people to use separate cabinet sims - I love IRs and will continue to use them in my studio, mind you, but part of the draw of this software is that it allows you to virtually tune your own amp sound that anyone can then have, and that means the cabinet needs to be distributable as well). So spend some time refining the cabinet sim if you can, maybe compare how it sounds to how IRs sound... The big thing that I've noticed if I were to critique it broadly is that the midrange is very uncolored with Cabinet Shaper, while the midrange with a real cabinet or an IR is extremely colored. It's one of the defining elements of the sound. I think this is a result of using low- and highpass filters to narrow the frequencies down - that will get rid of the hash that makes the amp sim sound nothing like a real amp, but it does not go the distance to making it sound authentic on its own.
You've got a High Amp and a Low Amp, very handy functions for dialing in the exact amount of tight-versus-loose gain you want there... How about a Mid Amp for further tuning?
If I were you I would think about removing the Chorus and replacing it with a standard Chorus; its operation reminds me of another plugin you made, I think you know which one, and it's pretty freaky; people looking for chorus might not really know what to do with it
I'm sure I'll have more suggestions going forward. I encourage other testers to take some time and do what I'm doing here - don't pull any punches, Ken is a big boy and he can take critique, it's like miracle grow to the guy and you'll see the product blossom and become lush before your very eyes if you take the time to give Ken feedback!
I know thats the only issue people are having. saying, hey man does that Chorus work, or I think I'm doing something wrong, I get no chorus sound. For a developer, in my opinion the worst thing that you can hear is they think they're doing something wrong. The user should always be able to turn a knob and something happens. But unfortunately The changes that happen in the signal path are not always audible until something else along the path changes. Once a user starts feeling like they are to blame for something not working you've just lost them. I studied software devlopment psych, Very interesting and important stuff. Like simple stuff, you want a user to take an action, or to strongly suggest they press a certain button, Make it green, The one you don't want them to press make it red. Simple stuff like that.
