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