Krell 450mcx mono/subwoofer connection

Your amplifier has balanced outputs, meaning that the negative speaker terminal has a full amplitude signal on it, rather than being grounded. So you should not connect the negative speaker-level input of a sub (which presumably is grounded within the sub through some relatively low impedance) to the negative output terminal of the amp. If you will be using two subs, connecting that wire from each sub to a chassis screw on the corresponding amp will most likely work well, and is what is recommended in the manuals for many REL subs.

If you will be using one sub which provides speaker-level inputs for two channels, as recommended in the REL manuals (when monoblock amps are being used) it would probably be best to connect that wire to a chassis screw or other ground point on the preamp. Although I can’t say that with certainty, as I’m not familiar with the technical details of the CAST interface you are using between the preamp and power amps.

In all of those cases the positive speaker-level inputs of the sub(s) would be connected to the positive output terminal of the amp for the corresponding channel.

Alternatively, given that the preamp’s CAST outputs are being used to connect to the amps you could connect the line-level inputs of the sub(s) to the preamp’s RCA outputs.

Regards,
-- Al

P.S: If you were to use a single sub which provides speaker-level inputs for two channels, the two negative speaker-level input terminals of that sub would most likely be connected directly together within the sub. So if you were to connect one of those terminals to the negative output terminal of one monoblock, and the other of those terminals to the negative output terminal of the other monoblock, you would be shorting the full amplitude signals on those output terminals of the two amps directly together. Which would probably result in self-protective shutdowns of the amps, or worse.

Regards,
-- Al

Al
Thank You  for your reply

Mark