connecting a sub to a 2-channel system

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