Help with hum

Since there is a ground connection on the interconnects, I don't see how connecting a separate wire as suggested would help. Plus I personally wouldn't do it because of possible electrical consequences that I don't really understand. Better safe than sorry. Good luck finding a solution!

It is independent of volume control and completely stops when the preamp is disconnected.

Just a guess but I would bet the ground loop is caused by one of the amps. When you have both amps connected to the preamp the ground loop circuit is completed by the ics.....
Just for a test try hooking up only one amp at a time to the preamp. Check for hum.

Jensen makes an isolator that can be installed between the preamp and the amps.

http://www.jensen-transformers.com/

http://www.jensen-transformers.com/an/an004.pdf

Been there.Save yourself some grief and borrow or steal another pre amp,plug it in the system and if the hum disappears you will have identified or eliminated the pre amp as the source.

Has it always hummed?

I think the manufacturers specs for hum on your pre are -88db below full power. Whether or not this results in audible hum on most systems I can't answer, what I do know is that there is audible hum with either of two tube phono pre's that I've owned and with my current tubed pre. I've come to just except it as part of the cost of going the tubed route.
An option, assuming that you have plenty of gain in your system, would be to call manley and ask them about reducing the gain on your power amp, this is usually an easy thing to do but I'm not up on the manley's. If you have a friend who is handy with electrical stuff they could make a very inexpensive couple of voltage dividers for you to play with as a way of determining whether reducing system gain seems like an improvement. Tweaking down the gain on your power amp obvisously won't eliminate the hum but maybe take it to a point you are more ok with. Good luck.