Low buzzing through speaks

Put a 10db attenuator between your amp and preamp.

@kingbr OK are you ready to solve your problem? I think the high gain of the XLR connection or amp/preamp combo gain is your issue. A 10 db attenuator will kill that.  You might have to use a XLR to RCA adapter to make it work in your application. I'm 99% sure that is what your issue is.

Be sure to read this: A Discussion About Idle Hiss – Upscale Audio

I am going to disconnect XLR’s and try the RCA’s first thing in the AM. Then I’ll try connecting the wire for the ground between the Mac and Sim…

If you have the RCA option, that will be 3db down and might solve your issue, if not see above. 

Wouldn't the XLR 10 db attenuator pad attached to each of my XLR cables work?

Yes, it would. 

BTW, I had this issue with my Mac402 amp. The attenuator did the trick. 

for every -dB of attenuation (5, 10, 20, etc.) that is used, you’ll lose that in volume when playing your system

In almost all cases you'll have plenty of gain to fully drive the amp.

I would hope that both would be 0.0V with no audio playing, or the volume turned all the way down.

That is very seldom the case. Volume potentiometers knock down the preamps full power output. Once the gain drops below the audible range there is no need for further attenuation. That is why the idle hiss can be audible, there is still gain being passed through the preamp.

So it would appear switching to RCS’s won’t make any difference, no?

The RCAs will be 3db down from the XLR connections. 3db might be enough to knock down the hiss a bit. 

Any update?

The old amp did not moan and hum, so I doubt that changing the whole home’s wiring is going to be easier than chucking in the old amp.

The two most common sources of new/increased noise when things were fine before, is an amp with more gain or a more efficient pair of loudspeakers.