You are caught in a bind of ohms, watts, and combo achitecture. I had similar problems, albeit at lower watts.
BTW, eveyone wants to keep weight and cost down, but at some point, if you want it to be loud enough, you have to have the watts and the cabs that can translate the power to sound. Buy used and get a dolly.
Unless I have misundertood, you wish to get a 4 ohm 4x10 to fit into your current (800 watts at 2 ohms) setup. Is that right?
After that, then what?
A superbly performing 4x10 used is still not super cheap.
Which single good 4 ohm 4x10 is enough when you replace the BRX212 with a head ro a rack?
I ask because if you think it all through, a 4 ohm 4x10 that won't need to be replaced will leave you a lot more options that you have now, but it presents the 2 ohm hassle all over again when paired with any other 4 ohm cabs. I know plenty of amps run a 2 ohms. I've just found that 8 ohm cabs can be shuffled into other gear configs more easily and flexibly.
IMHO you need to get out of the combo and 2 ohm configuration to broaden your choices.
The questions only you can answer are:
What do I want my rig to be?
What can I live with now while I save funds and look/learn.
What is enough? 'tis a tough question that probably can rarely really be answered, but a little overkill is way better that a little underkill.
BTW, JimmyM did offer good advice about EQ, flat or boost mids will get you more level. Also consider a compressor, it can help you max your level. However, in a volume race with bass verses 2 guitars and drums, you always lose.
Get the other musicians to downsize. Yeah....right!
Be sure you are wearing ear protection.
