I think a bigger practice space makes it more comfortable for you guys,,, and might help as you are a little further apart from everyone else... Like if you have a little room to move, you don't have to be wedged in between a guitar amp and the drummer.
If you really have your master maxed, and the input gain is clipping, and all else is flat, you should be loud. There might be something wrong with your amp. If you can get in a Guitar Center or something, take your head, and try to compare to another one. Back in the day when I played in pretty loud bands, I always used a 4x10 with about 300ish watts. If you guys are really loud, then you might need more. Some bands are just really loud. You also might be pushing the amp too hard, which is causing some distortion and compression, which is actually making you sound more quiet. Try backing off a bit and going for that big clean sound.
Also, spend some time getting to know your rig, and how it sounds in different rooms and situations. And ask your Guitar players to turn down a little bit. Just say,, until I can get some money for more speakers, can you turn down, just at practice. If they are cool dudes, they will understand
I'm not a fan of the MarkBass stuff. Had a LMII for a while, and I though it was kind of soft, didn't cut through, and I didn't care for the eq - which I think made it hard to hear. Never got the sound I wanted. I know you have the $$ issue, but maybe try something else. Any buddies with bass gear,, borrow some for one practice? Maybe you can work up a trade, for a different head, or trade your 4x10 with some dude looking to downsize his 8x10 rig. There are ways around the money thing. Post for trades on here and on CL. Ya never know, and it won't hurt anything. Also, maybe think about adding something like a VT pedal or SABDDI. You might find a cheep used one, and it might change the sound more to your liking, and that might help you hear.