Wow, wow, wow!
With the volume cranked in the top boost channel, this thing is a beast, it's alive - I love the way it feeds back so easily, I can feel it interacting with my instrument when I'm not even playing. The breakup when pushed can sound like an overblown brass instrument, or a trainwreck, it's just so gritty, abrasive and yet so harmonically pleasing. Very responsive - you can get all kinds of different sounds out of it by picking in different ways, and the distortion never sounds boring or generic - it always has a life of its own, it growls and snarls. It puts a modeling amp to shame.
Compared to my Blues Jr - The AC15 delivers in the areas the Fender is lacking. None of that flabby, farty, cluttered-sounding bass end, so much more focused. And beyond that it is just a totally different sound. Very compressed, very harmonically rich, and with a much harder edge than the Blues Jr which has a smoother sound.
Sounds great with every guitar I've played through it. I especially love the bite I get on the bridge pickup of a telecaster. On a semi-hollowbody with humbuckers it easily goes from Beatles to Zeppelin by fiddling with the volume control on the guitar.
Some of the sounds I can get out of this amp, with a couple stompboxes, have sent chills down my spine. Like airplanes crashing, or ghostly feedback like howling winds. Perfect with a slapback analog delay. I've also had great results with wahs and a boutique overdrive pedal.
This is the sound I've been looking for, it's inspiring to play through something that sounds so good.
My only complaint, and I know this is a common one, is that the reverb isn't that great, or maybe it's just not what I'm used to. I think I prefer the Fender's spring reverb. The AC15 has more of a 'hall' type reverb, which will find its uses though, I'm sure. So far it seems I'm going to want to keep it turned down very low, or off entirely. But I love having a tremolo to play around with.