I cannot use balanced connections to the amps because I need the low frequency separation at the subwoofer (so there is only the preamp-RCA-subwoofer-RCA-poweramp route or putting the subwoofer in the chain after the poweramps using the speaker wires (IMHO worse than RCA); this 80 Hz split yields a big performance improvement for the speakers similar to the effect of bi-amping. Not happy that the sub doesn't support balanced in-out for this purpose, but things are what they are ...OK. Given that I would suggest that you perform an experiment in which you completely disconnect the sub from the setup, and connect dac/pre to power amps via a pair of balanced xlr cables. If you don't have any suitable ones presently, buy a pair of inexpensive pro-oriented cables such as these Mogami's.
If that results in the noise issues going away, then buy a Jensen Iso-max, similar to those shown near the bottom of this page, configured and specified in consultation with them to adapt the rca outputs of the sub to the xlr inputs of the power amp.
Their transformers have an excellent reputation for sonic performance, and using one between the sub and the power amps would allow you to drive the fully balanced amps with balanced signal pairs (be sure to make clear to them that that is part of what you are trying to accomplish, so that you'll get the right kind of transformer), which will improve the performance of the amps as well as presumably resolve the noise problem. If that doesn't resolve the noise problem, then also add a transformer between the dac/pre's balanced outputs and the sub.
See this paper for some good background on interfacing balanced and unbalanced equipment.
Regards,
-- Al