Alright, so I have a Peavey pro bass 500, and I have a Peavey 4x10 cabinet to go with it, however, I cant use the cabinet up in my room, its kinda loud. I do however have the head up here, and a Fender Rumble 15 amp. Also my guitar amp, Roland Cube 30. So my question is, how would you go about connecting the head to one of the amps with out running it through the amps EQ?
Kramer Striker FR-424
Fender Starcaster
Roland Cube 30
Roland Mini Cube

Digitech RP90
Squier P-Bass (Insanely modded)
Fender Rumble 15
I don't think the issue is your cab. Your issue is the fact that you're pumping a bunch of watts into it. You can't really put it into a 15 watt Rumble because the speaker in that amp cannot handle the amount of wattage you want to put into it. I suggest if you need it quieter just use the Rumble.
you can link your amps using the "preamp line out" of the Peavey, into the input of your practice amp. but DO NOT use the speaker outs as inputs to anything other than a cab that can handle the wattage.
The other alternative is to take the 15W output from the practice amp and put it through the peavey speakers. this will give you the better tone from the speakers but with only 15W. I use my 20W peavey microbass through a 1x15 for practice and it sounds pretty good. You may have to do a mod to add an extension jack, but a competent repair shop will do this for you.

Just a thought