So, I have a bass combo amp (Peavey TKO 115S, 1x15)

and it has two jacks in it, one called "pre amp out" and "pwr. amp in" (I'm assuming pwr. is short for power)

so what do these two do?

Thanks.

Well, Im guessing you use Pwr. Amp In to plug your bass in, and pre amp out would be for effects? looping?

I dont really know but someone on this site must.

The pre amp out can be used for effects loop, or you can send it to another amp. or mixer board,
anywhere you want the pre amped tone of your bass to go...

 The power amp in is just that direct in to the power amp section.(part that supplies higher power signal to the speakers)..

  there are a lot of combination set ups.. you could get a real nice pre amp and the bypass the peavey preamp and just use the power amp  section. You could use the peavey pre amp and send the signal out to other amps too .. but usually , it is sent thru things like effects.. eq's. compression, delay/reverb units, signal enhancers (maximizer), phase units , ect. then the signal would come out of the output of those units back into the power amp section. (effects loop)

 hope this helps .

So, could I use my Peavey to power a cab by chance?

in the back of the amp, look to see if you have extra speaker outs... make sure the ohms match up, either

 4, 8, or 16..  The ohms number on the amp can be lower than the number on the cab...in other words you can run  a 4 ohm speaker out to a 8 ohm or 16 ohm cab, never ever run  a higher speaker output (from the amp) into a lower ohm numbered cabinet..... The thing about not matching ohms.. lets say a 4 ohm speaker out into a 8  or 16 ohm cabinet.. you not using the best amp efficiency you'll lose volume compared to a speaker out put to a matched cabinet input... ie.. 4ohm to 4ohm.  best match for amp efficiency.  or 8 to 8  ect.

So look on the back of your amp for AUX speaker outputs or something similar. check the ohms on it and make sure your cab is the same  or higher ohmage. .