Loud buzz with Marantz receiver preout to amp

Making progress: The amp has a clip that connects its “input” and “chassis” grounds. The manual states that while the clip should remain there in most cases, a ground loop can be helped by removing it. I removed it, and while a loud hum remains, I DO finally get sound from the Marantz’s preamp output — the only change I’ve made that’s done that. The hum seems to be improved slightly by reversing the AC plug into the power strip, as you mentioned, @jea48 — thank you. I removed all the other source inputs — no difference.

The Ashly makes a slight hum while it’s on that’s coming from inside its chassis. This is regardless of speaker output — it remains audible even with the gain turned all the way down, not from the speakers, from the amp itself. The hum sounds like the little version of what comes from the speakers when the gain’s turned up. The Marantz is silent when on — no buzz or hum coming from inside its case.

I wonder if getting an RCA-to-XLR box and the associated cables would improve the situation at all. I’ve been looking around the house for a ground cheater plug. May need to run to the HW store…

OK, it is my recollection that Marantz jumper plugs have an extra long pin to engage the internal connection or such. 

I had actually considered this, but it seems like you would end up with no sound, rather than a loud hum..., but who knows?

This is a Marantz Jumper.  Note how long the positive pins are.  You can readily find examples on ebay.  

Thanks, @toddalin — yes, I have four of those: One for each set of Preamp-to-Mains (front and rear). I removed the top two (front) so that I could plug in the preamp output to the amp. The mains are sitting empty — I tried them with an RCA cable between the rear preamp out and the Front mains in (as the manual suggests so that you can have the Marantz running power to the rear speakers). I don’t use rear speakers, but I tried it just in case, and it didn’t make a difference in the hum.

I am suggesting that perhaps your "RCA cable" does not have long enough positive pins to properly engage the signal through the jack.  Are the pins on your cable as long as shown on that jumper?

SOLVED! I had to run a spare speaker wire between the chassis ground on the Marantz to the chassis ground on the Ashly. No more hum! Thanks for the help, everyone.