Solved it! (Well, more or less).

I replaced the 5 XLR cables connecting the Pre to the Amp with RCAs, and now everything is as it should be. Honestly, I don't know what could have caused this, but I'm not going to worry too much about it. I use very short runs of cable (less than 3 feet), so it would seem there is no advantage to XLR over RCA anyway.

Either I had a faulty cable, or there is some incompatibility between my Pre and my Amp. I'm leaning toward the latter. I found an interesting passage in the Marantz 8801 manual here:

Quote:
AV8801 BALANCED XLR PRE OUT terminal PIN arrangement:

1 GND (Ground)
2 HOT (+)
3 COLD (–)
The PIN arrangement in this device uses the European method.
In the USA method, 2 is COLD, and 3 is HOT.
When connecting a device that utilizes the USA type of PIN arrangement, replace
the 2 and 3 plugs on one side of the balanced cable.


This seems crazy to me! So rather than spend any more time on it, I'm just going to be happy with my unbalanced RCA connections and leave it at that! But again, if anyone has any insight, I would be interested to hear it. Thanks again!