yeah definitely get another cab. another 8 ohm and unlock the full power. But I'd be inclined to encourage everyone to play at a more reasonable volume. Easier said than done, especially when everyone gets carried away.
My last band that I used to play with were ridiculously loud. Our drummer just used to beat the sh*t of the kit and the lead guitarist couldn't "hear himself", so know what you're saying.
Totally unnecessary in a practice environment. Apart from anything else, the room acoustics suffer massively when it gets too loud. It just becomes a soup of distorted noise.