I have been there and done that. Started out with several combos, like you.
(By the way, don't run that line out into the input of another combo. It will sound fine until it breaks something. All the technical details may contradict me, but I burned one up and still regret it.)
This is only my opinion, of course:
1) Ditch all your combos ASAP. You need ONE master volume control.
2) You Will Love More Watts. It is for the TONE, not the volume.
3) You won't overpower anyone with even a 1000W (@4 Ohms) rig, if you run it at 8ohms when you should.
4) You are very likely to overpower them while trying to crank a decent tone out of an underpowered rig.
5) Run one 8 ohm cab (preferably 4x10) when you want to keep it down, and two of them only when you want to have massive volume available.
6) Start with one 4x10. Make sure it is rated at at least 400W and runs at 8 ohms. 600W rating is even better. You can add a second one later, if you even need it.
7) Avoid 4-ohm cabs altogether. You want to be modular.
8) Avoid smaller cabs like 2x10s and 1x12s altogether too. You are playing Metal.
9) When you play shows that have "PA Support" for your bass, you usually need only one cab.
Again, that's just one way to go. You have a lot of other good advice here.
PLEASE WEAR EARPLUGS, ESPECIALLY AT A METAL REHEARSAL WHERE THE VOLUME MIGHT KEEP CREEPING UP AND UP. Keep them in on breaks too and you'll get used to them. Wear cheap foam ones until you can afford some custom-molded ones. The foam ones are much safer for your ears than most of the flanged ripoffs sold for 15-20 dollars. You can get a hundred of the cheap foam pairs for the same price. If I could go back in time to my broke rock and roll youth, and take just one piece of essential equipment with me, it would not be the bass or amp. It would be the custom-molded earplugs. (Come to think of it, I wonder if they'd have fit...)
)
