5x15 would be too small, I think. I presume that's 5' wide 10'deep. 10 x 10 would be better, but still crammed, and those walls will wreak havoc on the sound if you position yourselves with enough playing space. Cheap solution there would be if you can get enough egg crates to cover the walls. (Voice of experience, got in a lot of doodoo for all the staple holes in the walls.)
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"It's a poor musician that blames his instrument."
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You don't say where you're at. Some of the bigger citys have furnished practice rooms you can rent hourly. These come with amps, drums & PA. Usually 10-20 per hour. I've used Evolution Studios in Bellevue, WA and it's awesome for bands without a dedicated practice space. You might look into something like this in your area.
If you get into a space like that, respect your hearing - EVERYONE will need to wear hearing protection. The sound bounce will be brutal and damaging to hearing.
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