connecting a sub to a 2-channel system

My sister is coming for a visit. she is a budding audiophile and we will be having a listening session in front of my 2-channel rig. She is still very much a bass-head, and I want to hook my HSU VTF MkII sub into my 2-channel system, so I can show her what it sounds like with and without the big-booty-bass. My question is this: how to best hook the sub into the 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. Thoughts? Thanks in advance. Keep in mind this is not a permanent addition...!
If the HSU has speaker inputs, you can use to speaker taps from your amp. I find this to be an easier blend since sub and speakers see the same signal. It's how mine is hooked up.
I think that's how I hooked up the HSU to my Yami receiver back in "the day" (2009). The Yamaha had A/B speaker connections tho, the Rotel does not, so I'd have to wire both sets of speaker cable (quad 22l2s and the sub) into the same single pair of amp connections? I think that's possible, was planning on "dual wiring" the quads anyway. I use Audioquest type 4 cable, and there is still room inside the AQ bannana plugs for another twist of copper, but I would have to get longer runs of wire for the sub, which will go in the corner. You don't think the Y adapter trick would work?
I'm not a "y" adapter fan, but in some cases they work without sonic degrading.
I would not use the LFE input on the HSU if you are connecting it from the pre-outs of your DAC or Preamp.

The LFE input would bypass the crossover control on the sub since the LFE input on the sub assumes the source component you are connecting the sub to is doing bass management.

Since you are connecting the sub to an 'old school' 2-channel system, you'd want to connect both the left and right pre-outs to the left and right RCA inputs or use the speaker cable connections.

This way you can use the crossover control to set the cut off frequency of the sub so that it 'blends' with your main speakers.
Keep in mind that if you use speaker-level connections to the sub you don't need heavy gauge speaker wire, because you are driving the very high input impedance of the sub's amplifier (probably many thousands of ohms), rather than the low impedance of a speaker (4 to 8 ohms or thereabouts). Meaning that the cable will be conducting miniscule amounts of current. Even 24 gauge hookup wire would probably be adequate.
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.
Not sure what this means. I think you are referring to using the y-adapter to sum the left and right channel outputs together, and feed a combined mono signal into the LFE input. While Erik's comment about the LFE input bypassing the crossover is correct in most cases, it appears that your particular sub provides the option of switching the crossover in or out for the LFE input. See page 5 of the manual. However, although using a y-adapter to sum left and right channels together is done by a lot of people, with good results, IMO connecting two outputs together is not good practice, and in some cases will cause problems. See my post here.

Also, using the outputs of the DAC, which would not vary with changes in the preamp's volume control setting, would seem to be impractical.

So the choices that appear to me to be reasonable are, as Erik indicated, either using speaker-level connections, or using a pair of y-adapters to split the outputs of the preamp, with each channel connected to both the power amp and the corresponding line-level input of the sub. The low 60 ohm specified output impedance of your Halo P3 preamp should minimize or eliminate any adverse sonic effects that might otherwise result from driving multiple loads and multiple cables. But using speaker-level connections is probably the best choice, if practical, for the reason stated in MT's first response.

Best regards,
-- Al
Wow, thanks Al and everyone else...I think I'll twist some low gauge copper wire into the amp's taps and connect to the HSU's speaker inputs. Great info.
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
I'll try it tomorrow night with the sub turned off, see if that restores the Quads' ruler-flat mid-range. I feel like I am missing some detail in the mids, like the sub is covering it up or something. This is only temporary anyway, I miss the sub in my home theater too much! Funny how I used to crave bass when I first got into this hobby, now I just...don't!
I think I know what was wrong, crossover was not set correctly, now the sub and speakers overlap pretty seamlessly. Thanks all.
OK now that my sister has left, her visit ended, I returned the sub to the home theater system for our viewing of "snow white and the huntsman." I listened to my 2-channel system for about 2 hours last night and I think I miss the sub. It added some weight and presence to the music. Another thing: I use a computer desk in my listening room to work from home, I just move it out when people come in for a listen. I had to move it back in last night. There is a large 24" monitor that sits on this desk, and I think it really kills the imaging! Grr!