connecting a sub to a 2-channel system

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