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.

Thank you. I'll give them a call if I can't figure it out, but I think what I said above is correct.

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?

You got it right!
https://ogrforum.ogaugerr.com/fileSendAction/fcType/0/fcOid/43524898376238682/filePointer/4352489837...
Any combination will work.

Check out REL subs.  They connect through the speaker outs on your amp but only augment the base without involving a crossover.

Thanks!  I was actually just looking at one.  I need to test them out, but a sealed sub seems like a better move for me than ported.  I don't want to bass to over power, just something to fill out the sound 

Imhififan, yes!  Perfect picture.

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.