I have a C-EX too and it is pretty much hum free. I recently added front jacks and wired them directly to the input tube. Since I did that I am getting the weird hum problems that others have described... The amp hums even when it's off. It will continue to hum until I disconnect the rca's  from the front jacks AND remove the power cord. If either of these remain connected, the hum continues.

I purchased the amp new with the EX option, at the same time I purchased the CSP for gain with rock; both end of year after Decfest. Really only needed the CSP with bias in low position and high volume. I switch between the CEX and an F1, using the CSP as the Pre for both. I usually play either for about 1- 2 weeks, they stay on continuously during that time, that makes a big difference. The CSP stays on all the time. In other words, the amp has lots of time, but there was no hum from the beginning.
Joel

Bump

what seems to be the average hum levels of everyones EX modded amps?  I think mine's running about 4.5mv with the input tube in and 3.4mv with it out.    

That seems to be a bit excessive given the decriptions here.

Jason