In similar to you in that I have a big amp bias too, I've just downsized from a 100watt jcm900 to the tweaker 15watt.
Let me tell you this thing is LOUD! Remember that dividing the watts by 10 halves the volume, so the tweaker is only half the volume, or 3db quiter than a 100watt valve head.
Also bear in mind that the wattage is quoted as clean power, 5% thd on the tweaker. Flat out and with fully saturated output section it could put out as much as 30 watts.
I would have no probs gigging the tweaker in a mid size club, but If you need a clean sound you may want to go for the 40watt for increased clean headroom at gig levels.
Personally I just use dirty sounds really, so I have no issues with the 15watt.
Also bear in mind that you can pair a head with a more efficient speaker for increased volume.