ive never used anything but combo amps, and i need to clarify something...
i have a 500 watt peavey bam head. i want a 1200 watt carvin neodymium 4x10 cab (its on sale)
i dont think my head is big enough for the cab...can i use a power amp?
the smallest power amp ive seen is 1200 watts which will put the total output at 1700, overloading each speaker by 125 watts...

must i settle for a 500 watt cab? or can i get something a little bigger?

A 500w bass amp is LOUD, it's best to have a cab rated higher than the amp, it helps prevent speaker distortion.

For example, Zakk Wylde (guitarist, I know), runs a 100w head into two cabs with 4 200w speakers each, I'd imagine eliminating speaker distortion is even more important for bassist.

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Yeah there's no problem at all running a 500 watt head into a cab that can handle over 1,000 watts. The rating on the cab isn't supposed to be matched to the head, it's meant as a maximum. So in your case with the Carvin as long as you stay below 1200 watts you should be fine, not you need 1200 watts in order to use it.

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Bass Rig: Spector Performer DLX, Spector Performer Classic, Peavey Millenium 5 string-->boss tu-2-->Boss bass limiter/enhancer-->Boss bass overdrive-->boss ph-3-->MXR phase 90-->Hartke 3500a into a peavey tour 15"and an avatar 1x12"

Guitar: Frankenstrat or bc rich warlock (both with emgs 85 or 81 in the bridge and locking tuners)-->Pod Xtlive-->Tubeworks poweramp--> avatar 2x12"

For sale/ trade:
1973 original Fender Mustang Bass-$750
BC Rich Warlock (planetwaves locking tuners, emg 85 in the bridge batery in neck pu postion held in by plastic cover, dunlop straplocks, and plastic hardshell coffin case with purple interior)-$250
Ampeg b2r (with flight approved roadcase)-$350
Epi Goth Thunderbird (dunlop straplocks and hard gigbag)-$300
Pod XTLive with bass pack-$300