hooking up subwoofer to 2 channel tube amp

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. 

Neutrik Speakon plugs are solderless little gems I’ve been using for years with my 2 REL subs and pro audio speakers. I have the angled version in both my RELs so they don’t protrude so much, and anybody with 5 minutes and a small screwdriver can put these on pretty much any cable (I use Canare Star Quads for my subs)…far superior to binding posts.

It is also possible to run speaker level connections from the speaker terminals direct to the subs.  Depending on location of the subs, this might allow a shorter run of wire than from amp to subs.