compatibility...
 

should I run a 4x10 or a 1x18 with my 4x12? which of the two would be more compatible?

You should run another 4X12 with your 4X12.

Yes.
Actually I'd question my cab, amp and signal path because I can't think of a single situation where 4x12 wouldn't get you heard.

trust me, I'd love to... where can I get one for less than 200 bucks?

its more for running either side of the stage... we gig in quite a few places that don't run bass through the PA... and I'm in competition with 2 full stacks...

got to thinking about it... guess I'm going to go with my gut on this and just get a Little Mark 800... screw getting a second cab. The Firebass 700 is only pushing around 240 watts per side at 4 ohms... I think my problem is headroom or lack of it.

According to a TB review, it can get 700 watts at 2 ohms so you can run 2 4ohm cabs with it.

Maybe a 4x10 would work with the 4x12? Something to try out. There are lots of 4x10's out there for a good price.

No bass in PA huh. Is your stage 50 ft wide? If not don't even think about running cabs on both sides.

Stack a couple of 2x12 vertically or a Fearful monster or a pile of BFM units. If that doesn't work the band is too damn loud by far.

well the firebass isnt mine... gotta get a new head anyway... I think things will improve vastly when I get that markbass....

A new head is cool, but whatever you do, don't put 2 bass cabs that far apart. Bass is omni-directional, so you'll fill the room with one cab. If you did use 2 cabs, (identical matched cabs are best), they should stack vertically, in any case.

right on... I think I'm gonna try for just upgrading heads

Yes, matching if possible. With similar drivers at a minimum.

Very good point.

Upgrading's always good. But more watts takes a back seat to greater speaker surface area, greater speaker SPL, greater Xmax, greater air displacement.

You really want louder? Think an additional cab. Identical if possible. Certainly try for the same configuration. Driver specs as close as possible. And yes, stack them.