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. 

You need a sub-woofer with speaker level input:
http://www.gammaelectronics.xyz/images/stereo-review_sept_1995_subwoofer_f3.jpg
OR
a sub-woofer with speaker level input and output:
https://images.crutchfieldonline.com/ImageBank/v20160824170200/ImageHandler/scale/978/978/core/learn...

I see.  So you are saying I need something like a spade plug at the base of the out and keep my banana plug into the terminal itself so there are actually 2 speakers (one main and a sub) attached to the same place?  Do I have that right?

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