I've heard Ampeg V4's are VERY loud and very clean. The Cars clean sounds came from those. Only problem is they use rare power tubes (7027s), though I heard they can be converted.
My Bugera 333XL stays very clean even maxxed out at 120 Watts (yes, I have been crazy enough to take that thing all the way up to 10 on the master and channel volume), it breaks up around 8 or 9, but only if you have a monster humbucker in the bridge like an EMG 81 or a Duncan Invader, but a Mustang, Jaguar, or Telecaster will still run clean at that level.
