Subwoofer connection to older tube amp

Yup. That's it.

Unless you have an extra set of line-outs from your pre-amp. Then you can run RCA or XLR cables from the extra main/line outs from the pre to the line in on the sub.

Thanks guys. Didn't think I could double the speaker wires on the same inputs but makes sense now. Appreciate your help.

Use the sub amp and save the power from your tube amp for speakers only. Your tube amp might not like the load if you hook up speaker wires to speakers and sub. You can run line level to the sub and then line level out of the sub to your amps.

The other way to do it would be to use a Y spliter cable coming out of your pre-amp. Run line level to the sub and another to the D-90 amp. you can also buy rca splitters that are machined out of a block. Audioquest makes one.

http://www.nextag.com/AudioQuest-M22FHRD-One-Piece-79384484/prices-html

As explained to me by the main tech at HSU, the builder of my sub, the second set of speaker cables going to your sub from the main speaker terminals aren't drawing any appreciable power, they're just picking up frequency and amplitude information to be amplified by the amp in the sub. He said the gauge of the cables going to the sub doesn't even need to be the same as your main speaker cables since they're carrying very little power.