Differential Monoblocks with twin REL subs

Well put questions, and welcome to Audiogon!

Option 2 won't work, because the signals that are sent into the sub on the red and yellow wires (which are normally L & R channel signals) are summed together in the sub. Summing together the two signals in a balanced signal pair, which are inverted relative to each other, would cause them to cancel each other out within the sub, resulting in essentially no sound.

What you want to achieve with respect to the black wire from each sub is to have it connected to the circuit ground of the corresponding amp. Option 1 would work ok provided that chassis and circuit ground are connected directly together within the amps, which may or may not be the case. If you have a multimeter, you could determine that by measuring the resistance between an unpainted metal point on the chassis and either the ground sleeve of an RCA connector, or pin 1 on an XLR connector. If you do that, disconnect the amp from everything else, including AC power, before making the measurement. You would want to be precise in making that measurement, looking for at most a tiny fraction of an ohm, as opposed to say several ohms.

An alternative approach which would certainly work (based on the assumption that the preamp and amps are connected via balanced cables) would be to purchase a simple RCA plug, similar to this one, and solder the black wire from the sub to the ground sleeve of the plug (soldering the wire to the lengthy metal protrusion that appears toward the right side of the photo). Connect nothing to the center pin of the plug. Then insert that plug into an RCA jack on the preamp, and you will have provided the sub with a proper ground connection.

The yellow and red leads would be connected to the + output terminal of the corresponding amp, per your option 1. Losing out on some of the signal is not an issue; the setting of the sub's level control that you settle on will reflect the voltage range you are providing it with.

Re question 3, the approaches I have described can be applied even if the amp is not differential.

Nice equipment, btw! Regards,
-- Al

However, there is a post marked as 'Ground' on the rear of teh Pre-amp, so I guess I can assume that this would do the trick as well? Can I connect both subs' black wire toteh same pre-amp ground post?

Although in some systems that approach will provide reasonable results, I would not recommend doing that in this case, at least without first making some multimeter measurements to see if that ground point is directly connected to the ground sleeve of the preamp's RCA output connectors. And looking at a photo of the rear panel of the preamp, which I found via Google, my guess is that it is not. Depending on the specifics of the designs, the result could conceivably be a big-time hum. Or even if that direct connection exists, you might still get a slight ground loop hum and/or slight sonic degradation.

Personally, I'd get an inexpensive multimeter to eliminate some of the guesswork, and some inexpensive Mogami Gold Studio balanced cables, and proceed as I indicated earlier. I wouldn't be surprised if you were to get better sonic results with those cables than with your present RCA cables, even if the RCA cables are far more expensive.

Regards,
-- Al