Add Sub Woofer

I have KRELL Pre/Power Amp KRC-2 & KSA-50S with two KLIPSCH
Chorus, I would like to Add a Sub-Woofer but do not know how to connect in to my system.
I would appreciate for any advice.
Look at Vandersteen and REL. They are very friendly to analog stereo setups.
REL allows you to use neutrik speakon connector. This has the advantage of ensuring that the REL receives the same signal as the main speakers. But it does not use the power from your amp just the signal.
Personally, I'd put an active x-over like the Marchand or NHT (if you can find one) between the preamp and power amp. I'd also think about adding a digital room correction unit for the subs (SVS/Audyssey or Velodyne SMS-1) between the bass output from the active x-over and the line-input to the subwoofer.

BTW, I had a KSA 50s for years and it still ranks very high among my favorite SS power amps.

I had a KRC2 prior to my KCT. I would hook the amp VIA XLR (balanced) and use the RCA's to the sub if it has that type of connectors. Good luck, John
IMO, I would sell the speakers.Take that money along with what you will spend on a nice sub and upgrade to better speakers.Your speakers are your weakest link...