Question on PrimaLuna EVO 400 Amp: Fully Balanced (Differential), or No?

Well, it appears I spoke too soon. The EVO 400 amp & pre developed a nasty ground loop hum / buzz that no amount of troubleshooting could cure. Indeed, the steady 20 - 25 dB bug zapper sound persisted, even after every source and peripheral was unplugged, including the sub. As soon as the amp came out of 'safe' mode, there it was.  Anyone experience anything similar?  Figure out a cure (short of installing a full solar array & getting off the grid)? I sent 'em back today, and am rolling the dice that I'll have better luck with the hybrid integrated EVO 300 Floyd. Fingers crossed....

If the amplifier employs AC filaments for the voltage amplifier tubes, a cathode/filament short in a tube could cause that.

I know this is quite a while later, but I thought I'd follow up & close the loop. I ended up returning both the amp & pre to Upscale, who were very gracious about taking them back. They tested both, and could find nothing wrong. Before sending them back, I hired an electrician to go over our entire TH; he, too, could find nothing amiss. I did order a PL EVO 300 Integrated Hybrid "Floyd" from Upscale, and have been very (very) happy with it. All's well that ends well....

@zmann007 My dealer has that EVO 300 Hybrid. I've listened to it for many hours on multiple speakers and really, really enjoy it.  Congrats on the purchase!