I looked up the manual for your sub and confirmed the two (left/right) line level RCA inputs that you mentioned. These two inputs will solve your Y-connector problem at the subwoofer end.
Just connect your receivers sub cable to the left (or right) sub input, and then plug the sub cable from your pre-amp into the remaining input. They won't interfere with each other this way.
(FYI: the high level input connector is actually a Neutrik Speakon connector, not an XLR connector. Just mentioning it in case you ever decide to buy one)
That still leaves the problem of having to mix the pre-amps Lab outputs together without using a Y-connector.
I can't recommend the MCM Custom Audio 50-20795 stereo-to-mono signal converter because it uses transformers to mix the signals together.
Transformers have a tendency to roll-off the response at the deepest frequencies, and subwoofers only operate at the deepest frequencies.
Without any specifications to guide me on its capabilities I just don't feel comfortable recommending that product.
If you can find or make your own cable that uses resistors to isolate the Lab outputs before joining them together to one cable that goes to the subwoofer, that would work just fine.
Does your pre-amp sit close to the receiver?
If it does I still think you should try my earlier suggestion of unplugging the receiver while listening to your pre-amp to see if the timed buzz problem goes away. (It still sounds like a nearby Wi-Fi transmitter problem to me)
If unplugging the receiver does eliminate the problem, then you can then try obtaining some clip-on ferrite cores and attaching them onto your pre-amp input cables to see if that will also silence the buzz.