connecting a sub to a 2-channel system

My current thinking is to use a Y-adapter on a single channel coming out of either the pre or the DAC, then use the Audioquest sub-x cable to the HSU's LFE input. one channel from the DAC would be fixed, using a single channel from the pre would be variable.

You're welcome! A P.S. to my previous post, though. Upon re-reading your initial post I believe I misinterpreted the quoted excerpt, and you were not referring to summing the two channels into a mono signal, but instead just to sending the output of one channel of the preamp or DAC to the sub's LFE input, while continuing to send the signal for that channel to the next component in the existing signal path. That would be sonically inferior to providing both channels to the sub, though, so the conclusions in my previous post still apply.

Best regards,
-- Al

Corrected link to the manual for the sub. Not sure why the link in my earlier post didn't come out correctly.

Best regards,
-- Al

OK, got the sub hooked up. It's making the house shake as I write this. We'll see what my sister thinks, she'll be here Friday.
I used some old speaker wire I bought in the early 90's. Twisted it right into the AQ banana plugs at the amp. It's old, clear-coated, oxidized, flat speaker wire, supposedly 16 gauge. Playing with the phase, crossover and volume now trying to get it integrated. Seems to work fine, but my Quads sound different, I know the impedance of the sub won't affect my amp, but still...I guess the new sound just takes some getting used to!

Further listening proves that I have to turn the volume up louder with the sub connected to get to the same SPLs I was getting in pure two-channel. I have a decibel meter and mild tinnitis, so I watch this kind of thing pretty carefully.

The loading presented by the sub to the power amplifier is not the reason. To prove that, see what happens when the sub is connected but its volume setting is turned all the way down.

What you are perceiving could be the result of a phasing problem, perhaps in combination with too high a crossover frequency, or a placement problem, or a room reflection issue. Given that the sub only provides two possible phase settings (0 and 180 degrees), and you've probably already tried both, my guess is that it's a placement issue, which at some frequencies in the bass region is resulting in the output of the sub arriving at the listening position with improper phasing relative to the same frequencies arriving from the Quads, for both settings of the phase switch.

Best regards,
-- Al