@minnesotafats - I did what you did day one and it cost me a trip to REL for my right sub. There are many approaches like the network ($150 from ReL)… I have that w/ PSA standoff (free)… if you search you can read my posts with pics for the ADD… I got it working w/ REL and PSA network and stand-off. Paul is right when the BHK goes into standby. My solution is a L/R RCA ( just using the GND) from BHK input (assume you use XLR) to REL LEF/0.1 and CCW pot. I turn my music off, wait for REL to go standby, then BHK Standby and no hum. Or turn BHK to standby and listen to hum for 30 min and then no hum when REL hits standby… or you can go the REL network route… be very careful w/ the black wire… it is the first question they ask when you create permanent hum and have to ship it back… and their service is great and John is just like Paul… a great guy who loves tunes. Unlike Paul, he has a C1 Vette and I own one w/ 1,000 HP, a 61 we had great conversation.
I don’t like the REL network claptrap crap on my amps so I shut off my Roon in my music room and come back out in 30 min and shut off BHK. I also did a test for someone that found out the hum goes away after the REL auto-sense kicks in and RELs subs to to standby. Only S3 and above have auto sense.
So, clap trap works and so does my easy solution on both BHK250 and BHK300s
Just so we are clear S3 is Class D… same as CF sub…