This is not a direct answer to your question, but I'd offer what I have done with a vintage Mac system and a newly added REL S/510 subwoofer. The results have transformed my system with a new dimension of depth, and no hind of artificial or boomy bass. The subwoofer simply disappears, but the added depth and spacial sense is transformative. I have a Mc 2125 amp, C28 pre amp, XR 5 Mac speakers (along with the MQ 101 Environmental Equalizer and Speaker Control Relay Box).
The Rel S/510 is wired directly to my main speaker terminals on the Speaker Control Relay Box using REL's "standard high level connection cable." The ground wire from the Rel connection is attached to the "c" terminal of my main speaker left output on my 2125 Amp (the right would work as well) to prevent a very nasty hum. The Rel's are then adjusted for gain and Crossover frequency. It was very easy to set up. Hence in my system the Rel subwoofer is connected as "part of" my main speaker and receives the same signal from the pre amp, amp and speaker control relay box. Trying to use the remote speaker option for the sub, and running it as essentially an entirely separate speaker, seems to me to complicate things and perhaps would make it more difficult to dial the sub in correctly. I may be entirely wrong on this. Anyway, hope this helps in a small way.