My recommendation - don't do it. I simply would not try to make a premium two channel room that also would be for band rehearsal. I would find another place to rehearse. You may not have that option, but if you do, I advise you don't mash sonic attributes of the two together.
If you go for the band rehearsal sized space, you will be building a cavern, not a room. The entire experience, including what gear you will need to run to make such a space seem appropriate will differ. Are you ready to pony up perhaps triple the money you have invested in audio to get the results appropriate for the space?
It seems like you have lots of room to work with; why not build two rooms appropriate for their purpose? You can use a common wall, but use different construction techniques for each on other walls. You can send me the consulting fee if you do so. ;)