You mean from the preamp to the sub.
yes (it’s an integrated, so I don’t have a separate preamp)
You need to have a look at shops that sell equipment to musicians. I’d recommend a straightforward transformer so that it’s passive. The one I bought years ago was by Xitel and it was a 1:1 transformer that didn’t reduce the signal significantly down to about 20hz.
Here’s what a transformer manufacturer says:
The one I bought had two channels and RCAs in and out.
UPDATE
This is the sort of thing:
thanks.
I really appreciate all these suggestions, but it still feels like I’m treating a symptom and not fixing the problem.
I still don’t understand why the REL hums when the SVS (also a three-prong grounded unit) did not.
Back of SVS sub:
As others have said, the problem tends to be because subwoofers often have an earth pin connected. I’ve had exactly the same problems (it seems that your sub won’t take speaker level connections, so unless you can transmit wirelessly you’ll need to either lift ground or isolate with a transformer). Try a transformer but get one that will allow low frequencies to pass without significant reduction.
What country are you in? - there must be music shops (or Amazon?)
Here’s another one - it’s called a DI box:
USA. There are plenty of places to get additional boxes/adapters/plugs/converters. But I really think for this system, the cambridge subs are plenty adequate, especially if I get a second X301 to replace the smaller X201 that is there now.
I know it’s not up to the audiophile standards that most of the folks here require, but in this space, it’s gonna be fine.
I’d still try giving REL a call in case we are all missing something. They’re experts with their own products.
As to why some subs behave different from others with respect to ground comes down to their amplifier and power supply design. If the design has ground connected to the negative RCA pin and your earth connections are sitting on different circuits, a ground impedance difference will cause current to flow from one device to the other causing the hum.
While less likely to be a problem, moving up in your current line doesn’t guarantee you won’t have the problem with the higher end device either–especially if they changed the power or amp design in a way that causes hum for you.
Good luck!
FWIW, I have three REL subs., all connected with the Speakon cable. No hum issues. IMO, your problem is not an “REL problem”. I know that does not resolve anything for you. Good luck figuring it out.
Again, I already have an X301 (8") paired with an X201 (6"5") in the system. No hum from either unit.
That was always an option, to simply get another X301 to create a matched pair. A much cheaper option, too. But I wanted to see if the RELs were all that.
They sound fine, but no better than the Cambridge Audio subs.
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I believe a lot of us have had a grounding issue over the years with all kinds of gear. Even if you figure out your issue, will you be happy with the REL? I believe your best option is to return the REL sub (if you can) and buy a Cambridge.
The hum people experience with the speakon connector is when people use amps without a common ground. If your amp has a common ground then you can use the speakon and should work flawlessly. If you have an amp without a common ground then you will probably have a hum. Personally I have M700 amps and they require me to have an additional rca cable connected to ground the sub but its not really a problem as you just connect any rca input on the amp to the .1 LFE input on the sub and that cable grounds the sub. Since you already have the rca cables ran to the sub id just connect that rca cable to the LFE input (make sure you turn the LFE input volume to off) and run the speakon connectors for the signal. REL’s high level input has always sounded much better to me and I think you may notice an improved sound using them. Let me know if this solves your problem.
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I can’t offer you any other solutions but there’s an important point missing here. From the quote above I take it you have evaluated the sound quality of the RELs, so do the RELs sound significantly better than your original subs? If yes, is it worth the hassle of solving the hum problem? I get the impression you’ve already answered the questions, but thought I’d ask anyway.
Yah, this is from yesterday:
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Geez, guess I need (better) glasses! That’s what I get for reading on my phone. Looks like you have made the right decision!
eeeehhhh i don’t know what I’m doing. 
I sent Music Direct a message over the weekend about it and they messaged me back today to call their customer service folks. So I did.
If I’m not doing the high-level speakon connections, they suggested the cheater plug, like some of you did. I expressed my concern about the safety of that, but the guy was pretty clear that it was no big deal.
So I unboxed the things again and set them up, and yeah, I’m trying it with a cheater plug and yeah, no hum.
Anyway, I also have a message into REL to see what their feeling is on running them this way. They do sound good.
If I get some time this weekend, I might try to run the speakon connections, just to see if I can discern a difference. My guess is that I won’t. 
Further updates as events warrant.
PS: I do have to say, the bright white LED on the back of the sub is annoying as all get-out – especially as they are fairly close to a white wall. When it’s dark, it lights up the whole room.
Do you folks cover the LED with something? If I keep them, I definitely will.
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I normally use electrical tape but it tends to leave the sticky residue.
I used the 5mm version of these in blue on mine, they are for model cars.
I just checked and they seem to be out of stock, but maybe they dont keep their website up to date.
Maybe this instead? Or try any hobby store
If you don’t mind a bluish light use painters tape less residue than electrical
What light? I thought audio people listened with their eyes closed.
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yes, this is true.
but the silly light is on even when the music is off.
At least I have a nightlight if I start wandering around in the middle of the night looking for snacks
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