Schiit setup troubleshooting

I checked my speaker<->amp connections last night and it turns out that they are connected correctly. I removed the Chord Qutest from my system and inserted a Schiit Yggdrasil in its place, connected to the Freya S via balanced XLR. Now the system works perfectly (fully balanced throughout). Perhaps the Qutest is the issue - I will need to test that separately this weekend.

I check my speaker<->amp connections last night and it turns out that they are connected correctly.

Is the sub still connected?  And can you indicate the specific make and model of the sub, and describe exactly how it is connected to the amps?

In any event, glad you have made some progress.

Regards,
-- Al

I actually disconnected the sub completely and this did not solve the problem - it is a REL T5i.  

What had you been connecting the black (ground) wire in the Speakon cable to?

The manual for your sub does not provide a specific recommendation for connecting a single sub to monoblock amps having balanced/differential outputs, stating that they "strongly advocate" using two subs with monoblocks (whether balanced or not). However based on their recommendations for other applications of your sub, and on recommendations I’ve seen in other REL manuals, when you reconnect the sub you should first try leaving the black wire unconnected. And if a hum results you should connect it to a ground point on the preamp. Such as the ground shell of an unused RCA connector (using an RCA plug with the center pin unconnected), or perhaps to a chassis screw.

In any event, the black wire should absolutely not be connected to a red output terminal of either amp, and probably not to a black output terminal either.

Regards,
-- Al