Big G wrote
I absolutely love the look of the AC4's, and had seriously been considering the one on the tree at present, but I just remember my experience when I bought the AC15 that the AC4's lack of a gain control was somewhat limiting, and left me feeling a little unsure I'd be able to tweak to get the desired level of grit on the sound.
A bit late in the discussion, but I thought I should just point out...a little known fact, but there are actually two versions of the AC4. The AC4TV is the one you've tried with volume, tone and attenuator controls (which they also make a head of) and is white. The lesser spotted model, the AC4C1, is blue in colour and has an extra preamp tube, gain and master volume controls and a two band EQ, but no attenuator. I'm not implying you should buy either of these - that wouldn't be allowed of course. Just FYI...
As for your needs, failing a small valve amp, I would suggest something DSP based along the lines of what Alan has been suggesting for the sake of versatility. I would go with software rather than a hardware unit though if it's only going to be used in the studio - assuming you have a good DI and audio interface. The money you save could be put towards a good amp later on for live playing.
Or you could ask for your AC15 back... ?