Weirdly Specific HUM issue

That actually makes sense.  Now disconnect all the inputs to the preamp. :)

Ground loops are caused by alternate current PATHS not sources.  Are your monoblocks plugged into the same outlet?

Try a HumX or a cheater on the Freya.

Thanks for the replies - I forgot to mention I am testing with no inputs currently connected to the preamp. Also, the monos are not plugged into the same receptacle (in fact the right channel is disconnected completely).

I do have an iPurifier plugged into the same receptacle as the power amp, I’ll try switching it to the preamp receptacle.

Try RCA connectors. (Shielded)

unplug a hdmi from your cable box or tv, this was our problem, the hdmi caused a tweeter static sound, I unplugged everything, checked every piece during connecting, first rca from amp to pre gave me the tweeter static, ….scratched my head,…….unplugged my TV connections, BAM, solid silence, it was the hdmi all the time, now when I listen to basement system, I unplug the hdmi, and I’m in audio bliss.

upstairs was different, we live just a couple miles from a radio tower, and would get the broadcast through my tweeters, enough where it could be heard from halfway to couch,…..tried my XLR, 2 different rca cables, then bought a pair of the Belden from the BJC, company, BAN, silence, it seems their cables are shielded better than the 2 pair of Audioquest I use.

bought couple more pairs of the Belden s, and it sounds better minus the CD player, which I swapped out Belden for the Audioquest Quartz rcas, that added some better midrange and treble.

considering going Groneberg for cd and DVD player, as I’ve heard good things about them.

I use the Groneberg in mnasement system from amps to preamp, they are a nice cable. 
 

those bcc’s are very well shielded. Plus the waf factor of Kelly green color rca’s. Adds some visual snap with the vibrant colors , other than black.

the Audioquest are nice, in a medium blue and a light blue turquoise color.

it just looks better.

better sound,…no, but visually appealing to the waf!

Because the grounds are common. You need to isolate the piece that is causing the hum. Unplug one item at a time until the hum disappears. Like musical chairs, the last one standing is it.