Powered subwoofer connection?


Are powered units available that run off your amp?
Or are there volume adjustments on non powered.
I have only one set of output terminals on my amp so envision adding second speaker cables using bananas.
Want to augment bookshelf speakers economically, experimenting.
ptss
Post removed 
"It may also be possible to run a set of speaker cable from the amp to the preamp."

oops, I meant to say... run the speaker cables from amp to sub.
CHECK OUT REL SUBS - besides the LFE input they use a Neutrik connection which attaches directly to your speaker outputs of your amp...this works very well for integrating with the mains.