hooking up subwoofer to 2 channel tube amp

You need a subwoofer with speaker-level inputs. Then you simply run wires from your amp's binding posts to the sub's binding posts. This is actually a superior method of integrating a subwoofer vs the LFE sub outputs found on AVRs. 

Just a few brands that have this feature are Rythmik, SVS, KEF, and Golden Ear. 

Call Crutchfield, they'll be able to explain it all.

Keep in mind that some subs, including RELs, use the Neutrik Speakon connector for speaker-level inputs. Thus, you can't just use basic speaker wire. Other brands, such as Rythmik and KEF use regular 5-way binding posts. IMO, the KEF Q-series, sealed-box subs are sleepers and offer great bang for the buck.