I agree with the several people who have suggested that the noise issue, which does indeed sound like ground loop noise, probably originates within the system. Ground loop issues are usually caused by interaction of three factors: Certain characteristics of the particular equipment (specifically low level leakage paths that may exist between ac and chassis, in the components' power transformers and associated circuits); how the equipment is interconnected (balanced or unbalanced, and interconnect cable shield resistance if unbalanced); and how ac power is distributed among the different components.
Looking at your system description, what primarily strikes me is "Piega P1 Mk ii active subwoofer (RCA loop-through and cut-off at 75Hz)." Unbalanced (rca) interconnections are inherently prone to ground loop issues. Does this statement mean that you are NOT using balanced connections to the power amps? If so, I suspect that the problem will go away if you connect the dac/preamp directly to the power amps via their xlr connectors (which you should be doing anyway with that equipment, for best sonics), and connect the sub to the power amp outputs via the high level inputs which the sub appears to have.
If I am not interpreting the setup correctly and you are presently connecting dac/preamp to power amps via balanced cabling, try changing the sub inputs from rca to hi-level anyway. ALTHOUGH WITH A FULLY BALANCED AMP, IF THE SUB'S HIGH LEVEL INPUTS ARE UNBALANCED YOU HAVE TO CONNECT BETWEEN THE AMP'S RED SPEAKER TERMINAL AND AMP OR PREAMP GROUND (CHASSIS), NOT BETWEEN THE AMP'S RED AND BLACK TERMINALS, OR YOU MAY DAMAGE THE AMP.
If none of that helps, or isn't applicable, temporarily eliminate the safety ground connection of the sub's ac power plug, using a cheater plug. If that solves the problem, consider getting an audio (not power) ground isolation transformer from Jensen Transformers to use in series with the rca cables to the sub. That would allow you to do away with the cheater plug, which as you probably realize creates a small but non-zero risk of electrical shock if an insulation fault were to develop in the sub.
Regards,
-- Al
Looking at your system description, what primarily strikes me is "Piega P1 Mk ii active subwoofer (RCA loop-through and cut-off at 75Hz)." Unbalanced (rca) interconnections are inherently prone to ground loop issues. Does this statement mean that you are NOT using balanced connections to the power amps? If so, I suspect that the problem will go away if you connect the dac/preamp directly to the power amps via their xlr connectors (which you should be doing anyway with that equipment, for best sonics), and connect the sub to the power amp outputs via the high level inputs which the sub appears to have.
If I am not interpreting the setup correctly and you are presently connecting dac/preamp to power amps via balanced cabling, try changing the sub inputs from rca to hi-level anyway. ALTHOUGH WITH A FULLY BALANCED AMP, IF THE SUB'S HIGH LEVEL INPUTS ARE UNBALANCED YOU HAVE TO CONNECT BETWEEN THE AMP'S RED SPEAKER TERMINAL AND AMP OR PREAMP GROUND (CHASSIS), NOT BETWEEN THE AMP'S RED AND BLACK TERMINALS, OR YOU MAY DAMAGE THE AMP.
If none of that helps, or isn't applicable, temporarily eliminate the safety ground connection of the sub's ac power plug, using a cheater plug. If that solves the problem, consider getting an audio (not power) ground isolation transformer from Jensen Transformers to use in series with the rca cables to the sub. That would allow you to do away with the cheater plug, which as you probably realize creates a small but non-zero risk of electrical shock if an insulation fault were to develop in the sub.
Regards,
-- Al