XLR to RCA conversion

You can search   "Earthquake Sound ISO-200 Hum and Buzz Kleaner 600 Ohm Ground Loop Isolator with RCA to XLR Signal Conversion"   on Amazon.  They are $36.95.  I can't say that it will solve your hum problem because those can originate in many places and the source can often be difficult to pinpoint, but if it doesn't work you can always return it for a refund.  I hope this helps.  Best of luck.

Did you try lifting the ground on either unit with a cheater plug first to see if that eliminated the hum?  Not a permanent solution but a test point.  For XLR to SE conversion units look first to Jensen before car audio products.  Also reach out to Ralph and see what he recommends.  

@jackd yes, I am communication with Ralph.  He recommended lots of ideas which I tried.  His most recent suggestion is a Jensen transformer, but the specific model he recommended isn’t available.  That’s why I posted.

He recommended the Jensen PI-2XR.

I am wondering if the PC-2XR will work.

Maybe a longer-term solution, it seems a bit of a shame you’re running a fully-balanced preamp into a single-ended amp.  If I’m you I’d be saving up for a good fully-balanced amp from the likes of Atmasphere, ARC, VAC, etc.  Just my $0.02 FWIW.