software

If given the right speakers/tubes can 15 watt tube amps remain clean through a PA system? Not sure if that makes sense, but basically I want to know if a 15 watt tube amp can have enough headroom to play w/ other guitars/drums. No matter what has to be done, new speakers, new tubes, run through a PA, whatever.

Through a PA is no prob...if your PA has enough power. After all, some folks will just run a POD through a PA.
The only problem I'd be concerned about is being able to hear yourself on stage if you have a hard-hitting drummer.

yes i think a 15w TUBE amp would work fine with a decent pa. i would probably get it off the ground and closer to ear level

Sure thing! (Assuming you've got a good clean to start with )

I had a 20watt DST that had respectable headroom without being PA'd or mic'd up.....just traded it for another louda$$ 20watter (Fuchs ODS Custom20 ) which has GREAT headroom for a little guy.

I would imagine a PA or mic would work perfectly without destroying the joint

I used a small Boogie Studio 22 combo with a huge Gospelchoir, just had it mic'ed and my sound send to a sidefill, no troubles

I used my Fender Blues Jr. with a Les Paul when I played with a 15 peice jazz band (5 saxophones, 3 trumpets, 3 trombones, piano, bass, drums, and me on guitar) with absolutely no monitor help. It kept up. That's all I have to say about that.

Well I used the Boogie for clean
Had a Kitty Hawk Quattro for drives.