You need stage volume...bottomline.If the house P.A sucks(or yours)then You might have to rely on your amp for stage volume...and no 15watt amp would cover that.If there are decent monitors,then you could get away with a nice sounding low-watt amp.

The chest pounding punch that roars out of good stacks is impossible from 1x12.4 to 8 12's push major air.

Tube amps usually sound there best at high volumes,so a low watt tube amp can attain a great sound at a much lower volume.Solid states happen to sound good at most any volume.If you dont want to lug a big stack,then a 2x12 is fine,and if your sure theres enough monitors you could bring a real sweet practice amp...the stage volume and tone will suffer though.

If I'm listening to a song on an album,the chest thump from 4x12 cabs dosn't have any relevance.
It also amazes me that so many people dont know to turn there stack or combo towards the drummer instead of facing the crowd.The audience really dosnt hear your stack.They hear it pumped in the P.A...theres no need to turn it at the audience,although,if you have a wall of 4x12/2x12 cabs,then it wouldnt matter much.