Tones - are they convincing in terms of modelling the "real thing"?
The modelling is very convincing right down to the signature hiss of the various makes. As for tweakable, eg VOX AC30 (1 of the 10 amps modelled), you can tweak the regular front panel, Normal, Brilliant, Bass, Treble, Tone Cut, Tremolo Speed, and Depth. You can also tweak the the tubes at the back of the amp, power supply, variac, sag, response and bias. On the speaker side you can adjust mike type and placement to find the sweet spot or build your own wall of speakers. Conclusion I would never have the money know-how or time to build what can be modelled.
can you go way overboard like taking an AC30 into hi-gain?
"The amps that go up to 11 are much better than ones that go up to 10" - This is Spinal Tap
Well you can drop in a gain booster before the amp to go to 11 and beyond. You can also do the Brian May and chain 3 AC30 together 1 parallel and 1 series and build a wall 4 x 12" cabs.
what is your favourite set-up using GR
I'm kind of spoiled for choice here, depends what I'm playing - E.g. For my blues I like to use a Fender Tweed, mic the cab at about 5cm and and add a small amount of quad delay, some times kick in the a cry baby.
What are the dynamics like in terms of sensitivity to attack, compression when driven hard, ect?
Besides being able to adjust the amps dynamics at the back there is also a whole bunch of compressors, filters and
equalizers that you can drop in to get the desired response. What you don't have to do is wait 30 min for your valves to warm up though ?
What powered speakers would you use/recommend with GR?
As big and as dynamic as you can afford. One thing I have learned in the amp sims business is two speakers are better than one since a lot of the dynamics and effects are stereo. I use a compact PA, Yamaha Stagepass @ 250 watts a channel for practices I usually will take only the one speaker with the mixer stashed in the back