Just a thought buddy.  Have you tried different cables??

power or signal - I guess I should try both.....

-m

Does the other one hum with the RCA disconnected? Does this one hum and turn on with the switch set to Powerlink instead of RCA?

Hum and noise picked up by the RCA input is enough to turn the speaker on if it's of sufficiently high amplitude, if one speaker does and the other doesn't I'd say something is wrong with one speaker. Now, which one?

the other one appears to be working fine

the hum goes away when on powerlink - but I have not left it on Powerlink mode long enough to see if the on/off is still an issue - part of the problem is that it is very intermittent

-michael

I experienced the same problem with my Beolab 8000's when they were connected via several RCA cables (about 15m) to my iMac. After swapping both the cables (not to expensive ones (20€) but the difference being gold plating and oxygen freeness or something like that) the problem disappeared. 

Have you tried to swap the RCA cables between the speakers to se if the problem is cable related (the other speaker would then have the same symptoms...).

tried different power cables - the hum seems to be there regardless of RCA or Powerlink (L/R) - I think louder when plugged into my pre-amp (via RCA) -and I think the hum is even there when playing music......probably the hum (feedback?) that is tripping the signal relay?

:(

-m

When you switched channels between the two, did you do it at the speaker end or the preamp end? Meaning, did you completely swap cables between the two speakers or just swap cables at the preamp?

FIrst thing is to make sure you don't have a bad cable, completely switch cables. If it's still there, the hum, it's in the speaker somewhere, though Powerlink thing is odd. Should at least narrow down the issue.

completely changed cables (and channels)

but the hum is also there even when there are no signal cables connected - meaning, only power to the wall (and thru a small and reasonable power strip and outlet - I guess I can change outlets to make sure it is not a grounding issue???)

thanks for your comments and ongoing help

-michael

plugged the mains into another outlet and that seems to have improved the problem (for now) - will keep you posted

maybe just not grounded properly?

-michael