Help! High level subwoofer input on a class D amp introduces ground loop.

I know my opinion diverges from the seeming majority of swarm sub guys on this forum, but I believe that (1) subs are very important for stereo music, and (2) connecting subs via their high level inputs is preferable for stereo music.

In my limited experience, the difference between high and low level inputs isn’t subtle. Through the high level input, the sub disappears into the mains. Full disclosure, my data set consists of two different, low end subs. So, yes, I am extrapolating from my limited experience to conclude that high level connections will also be best for other subs as well. To speak to the swarm guys, I do understand that multiple subs produce smooth sounding bass; I just want my bass to be well integrated too.

The wrinkle with regards to the high level input on a class D amp is the introduction of humming from the sub. I notice that when I set the sub to auto (where the sub turns itself off after X minutes of non-use), the sub doesn’t turn itself off anymore. It remains on with a slight hum now. In fact, the sub was on all night and seems to have knocked out my internet, which was on the same outlet as this sub. The internet promptly came back when I turned the sub off manually.

My sub is not a REL, but I can see that REL has instructions on how to use their high level connections with class D amps ( If I understand their article, REL recommends achieving ground either by running RCA from the preamp to the sub or attaching the (-) speaker cable to the turntable ground like the ones found on the back of NAD and Peachtree integrateds. I currently run a Peachtree, but it is an older model without the TT ground.

Correct me if I’m wrong, but here’s my understanding for connecting high level to a sub with 5-way binding posts on an integrated that doesn’t have a TT ground: run speaker cable between the amp and sub for the (+) of the R/L and don’t run anything for the (-) of the R/L, then run an RCA connecting to the (-) on the pre and sub. How’d I do?

I’ve never used a sub or subs in my system.

The speakers I’ve used over the years have all been spec’d down into the mid to upper 20s of Hz, with my present speakers (Daedalus Ulysses) being spec’d down to 28 Hz +/- 2 db, and with the rolloff below that frequency being gradual. And I use the digital signal processing functionality provided by the DEQX HDP-5 I use as my preamp to introduce some small and narrow equalizations which mostly compensate for what I’ve measured to be fairly minor room-related peaks and valleys in frequency response.

Some users of Daedalus Ulysses speakers use subs with them, but my impression is that most do not.

-- Al

@ghnrbob Interesting. My had a high pass filter, where the speakers connected to the sub and only received the higher frequencies. The sub I now use (an older Jamo) connects like a REL where the speaker receives the full range of frequencies and the sub supplements the lower frequencies. Both subs integrated better via speaker cable than when connecting via RCA/Coax. 
@millercarbon I love the pictures you have for your system. I imagine it sounds fantastic. If you could only run two subs, would you run them high or low level? Also, how many subs do you run? 4?
@almarg I'l have to read up on your speakers. I hear equalizing software provides real benefits for music.