well, it kinda depends on your musical needs.. I say if you like the sound you've already got, then why not a 50 or 100 watt marshall MG.. if not, and you need great clean sounds, then how about a fender fm212r? (warning though, it's INSANELY loud! :p) and if you need good distorted sounds, then why not like a randall RG75?

I think you'll be surprised that there is a big jump in volume from a 15W amp to a 50W amp.. even if they are both just combos.. trust me, I was using a 15W amp for gigging for a few weeks.. and it sucked.. so I went out to try and buy the loudest thing I could afford (fender fm212R), and it ended up being too loud! now I either use my 30W line 6 spider II (solid state), or my 40W peavey XXX (tube) for gigging, and they both have a good volume, the peavey just has alot more headroom and can get even louder then my fender could have, I just don't put it that loud for gigging.. with a solid state though, turning them down won't really work, since there is barely any volume when the knob is at 1, and then about 90% of it's maximum volume when the knob is turned to 2...