Rel Stadium 3 Humming

I am having the same trouble most are having with my Rel Stadium 3. I have a humm when the amp is off. I am running a Pass Labs X350.5 but mine has no grounding pin on the back like the new ones do and mine is 2 years old. I am also using a Pass X0.2 preamp. I have tried everything. Grounding it at the chassi screw, grounding it to a negative on an rca plug then plugging it into the amp or preamp rca, etc. When I ground it to the rca or the chassi the humm goes away about 85% but there is virtually no output from the sub. If I run it low-level with rca only it works perfect. Ken at Pass said it is bad to ground it at the negative amp terminal as it is hot as the rest of the terminals because it is a diferential amp and can cause the sub to actually smoke and catch fire, it has happened more than once he said. During large crescendos it can send something like 70amps to the sub possibly hurting the amp and definately destroy the sub. What to do? I bought the sub specifically for the line connections. I had this sub before with a regular X350 and no issues. Any input would be great, perhapse someone has a fix for this. Sumiko was no help at all, I mean none!!! Thanks Agonr's
Did you try using a ground cheater on the sub's amp power cord plug?
That was going to be my next move. Although it only happens when I use the Speakon cable. When I use the rca low-level all is fine but that is not how I want to run it. Funny thing, not really...
I have a pair of Pass XA160 Monoblocks in my main system and the Rel seems to work fine. Kent at Pass completely rules out the X350.5 as the problem though. Thanks for the reply by the way.
Your issue results from the fact that the grounds for the preamp and the amp float and do not connect to the chassis. What you have to do is to connect the two positive wires going to the REL (I believe they are red and green) to the positive speaker terminals on the amplifiers. It doesn't matter which goes to which channel. Then, get a cable splitter (e.g., RCA male to two RCA females, or XLR male to two XLR females if you're using balanced inputs). You should only this with one of the two channels. Plug the signal cable for one of the channels into one of the female ends of the splitter, and the male end into the normal input of the amp for that channel. Wire a male RCA plug or XLR connector to the black ground wire running to the REL. If RCA, solder the ground wire to the ground shield; if balanced, solder to pin 1. Plug it into the other end of the splitter. That should do it.

I've used this same setup for my Stadium III with two Mark Levinson 33H's and with a Boulder 2060. Yours should work the same way.
I do so apreciate all of your help with regard to this issue. It certainly seems to be a grounding issue of some sort. I took problem Rel upstairs to my bedroom surround system,3rd system in the house, hooked it up to my Lexicon surround receiver and it works great, no issues. The Lexicon does however maintain balanced in and outs but not true balanced and all the Pass amps I have are of course true differential balanced. I have ruled out an issue with the X350.5 or the Rel and determine the issue a grounding problem. Funny, the Pass XA160's have no problem with either Stadium III. All you guys are great and have important info. In a nutshell, the problem is a surge protector I use for all the Active Sheilding MPC's as I us all Synergistic Research Precision Reference speaker cable and balanced interconnects. I removed the MPC's from the surge protector (6 total) and nothing. I then removed the naked surge protector from the outlet it was sharing with the Rel and guess what, all is normal, no humm, or loss of signal. WOW! Thank you for all your help Agoner's, you are the best friends an Audiophile could have!!!