Two questions, is 15 watts enough power for a practice session in a garage with a band. And, if 15 watts isn't, what's a good amp that could? P.S I play heavy/death grindcore.
15 watts tube is enough for almost anything...

I've done okay with 15 watts Solid State, with a drummer and bassist.
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my mate's 15 watt tube can easily play louder than my other mates 100w bass amp!
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yeh thts plenty aslong as the drummer isnt mentally insane or the another guitarist has a bigger amp
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Eh... Maybe... If everyone plays quiet. If you crank it up though, it might start breaking up which sounds horrible with solid state amps.
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ya my 15 watt ss cant even be heard over the drums. my buddy has a peavey 80 watt amp and it has a multi ohm selector. so at 8 ohms its 80 watts, 6 ohms its 60 watts and 4 ohms its 40 watts (im not sure if those are the exact numbers but its like that) so when he plays in the band he can be loud and when he is at home he can be quiet without loosing sound qaulity. i have a multi effect pedal that i run through our bands 150 watt P.A. and thats how i play cuz i cant afford an amp right now.
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Naah, speaking from experience you'll be fine solid state or not. Mine coped even with a very loud drummer, a 100 watt bass amp, another 80 watt guitar amp, and something like a 60 watt amp that the vocals were going through.