Just a back to basics thought. Try either cheaters to unground the powered speakers, and/or the preamp, and see if the hum goes away in the unbalanced mode, or try using an extension cord to achieve a common ground. If this works the sub can be run normally from the preamp.
How do you use a sub with active speakers
I feel dumb about this one. I'm playing with a pair of Quad 12L Actives. I normally connect my REL sub using the high-level method from the amp's speaker terminals, but the amp is no longer in the system. The Quads are connected from the preamp's balanced outs. Do I use the preamp's unbalanced outs, via a Y adapter, to feed a mono preamp signal to the REL's low-level input? Are there other options?