What wattage are you thinking?
You know what would be seriously cool, and totally unique? A STEREO tube amp! Tube pre-amp, fx send, stereo fx return, and a power amp that can either run stereo or parallel mono. Head or 2x12 combo.
And here's the kicker: have one side of the power amp run 6L6's, and the other side run EL34's! Or put a switchable bias on both, so the user can decide: all 6L6, all EL34, or half and half.
If you've ever heard someone play with two different amps, and how MASSIVE their tone is, you'll understand the appeal of something like this.
Sorry, just brainstorming.