I use a REL with a tube amp and lifted the ground on the REL (This model, Q150E, has a switch for that). Humless! I also use a PS Audio Humbuster III on my power amps, so there's that.
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I have had a REL Storm III with a McIntosh MC275 mkIV. Was wired from the speaker terminals in the amp into the REL, as the preferred method by the manufacturer. So this is bass reinforcement at the lower levels. The amp still try to reproduce the whole spectrum. I was very happy with it.
Having said that, I replaced the ReL with two Rythmik subs and I'm happier, and I'm thinking it might be best to have a crossover between amp and preamp so to avoid the low freqs from getting into the amp and let the subs reproduce them.
I'm trying to get rid of a 60 cycle hum when the AMPS [emphasis added] are on...More detail is needed. First, are you saying that you are using a pair of monoblock amps? Also, what model is/are the amp or amps, and the sub? And am I correctly interpreting that you are connecting the sub using its speaker-level inputs, and that the hum is emanating from the sub?
Also, some of the vintage McIntosh amps have both a "gnd" (ground) terminal and a "com" (common) terminal adjacent to their 4, 8, and 16 ohm output terminals. If that is the case here, which one of them are you connecting the REL's black wire to ("gnd" or "com"), and is there a jumper on the amp between those two terminals?
If both of those terminals are present but there is no jumper between them, adding a jumper may fix the problem. That possibility is reinforced by your statement that the two-prong adapter made the hum much louder.
When connecting a single sub to two monoblock amps REL generally recommends connecting the sub's black (ground) wire to the chassis or other ground point of the preamp. The red and yellow wires would be connected as they apparently are presently, one of them to the 8 ohm tap of one amp and the other to the 8 ohm tap of the other amp.
I would NOT try that, however, without Gnd and Com being jumpered together on both amps. Connecting the black wire to preamp ground, or for that matter to Gnd on one of the amps, without the jumpers being in place would result in the sub seeing a floating input from each amp having no return path to the secondary of the output transformer. The result would probably be a huge hum.
The schematic appearing on the last two pages of the manual may help to clarify that.
I'm saying that if I am correct in assuming that the red wire from the sub is connected to the 8 ohm tap of one amp, and the yellow wire from the sub is connected to the 8 ohm tap of the other amp, then what you should try is connecting the black wire to a ground point on the PREAMP, such as a chassis screw. But you should only do that with the COM and GND terminals of each of the two amps jumpered together.