Lots of people use Y-splitters to send signals to their subs, including me. In the 43 years I've been an audiophile, I've never heard of a Y-splitter not working with a sub. Since both REL and M&K are popular, well-known, quality brands I am sure there's no problem with your subs. I would say that either your splitter is defective in some way, or there was something wrong with how you connected the sub to the splitter. A 50/60 Hz ground loop hum is probably what you heard. Are you sure the connections at both the splitter and at the subs were secure and pushed in all the way? If not, that would cause the 50/60 Hz hum.
I'll be more clear. It worked with the M&K, but not very well. Loud hum, but it has speaker level inputs, so that was easy. With the REL, I have tried a few different splitters with the BeePre and a Harman Kardon Citation IV preamp, which does not have a second RCA out. I assume in your 43 years, you did not own the specific REL model HT1003 MK1 subwoofer and Citation IV model preamp? The splitters or hardware were not defective. Yes, I am sure the connections were secure. I ended up using a speaker level RCA adapter which has 6 extra wires and a big plastic box, and that works, but doesn't seem clean and I do not want a plastic box hanging off the side of the amp. I am currently using a Citation II as a power amp, and the box is behind the amp in an enclosed cabinet, but with the Kaiju, I will be exposing that and finishing it nicely, so the exposed, hanging car audio jig is a poor solution for an all around expensive system.
The REL is UK made I believe which is probably the first issue.
I was asking if it were easily possible to add a second set of RCA outputs to the BeePre instead of doing the whole DC conversion with balanced outputs (which I can connect XLR to RCA cables I assume) because I have read the sound can change significantly and I would like to use the preamp stock for a while while breaking it in before deciding if I want to change anything.
Thank you for your time.