First, do you hear the hum ONLY on phono? If so, I agree with the notion that it most likely has to do with grounding between your preamp, tt, and tonearm. I am having a similar problem with my newly set-up Technics SP10 Mk2/RS-A1/Ortofon MC7500 rig. I normally run a Lenco/Triplanar/Koetsu combo that has absolutely no hum issues. With the SP10 et al, I get a volume-dependent hum, probably just like what you are experiencing. The SP10 is in a slate plinth which is absolutely not grounded. So far, grounding the RS-A1 tonearm to the preamp (an Atma-sphere MP1) did no good. My next plan is to ground the SP10 to the MP1. The MP1 is a true balanced circuit, and I have never had hum issues of any kind before, so I think I've got to blame the new rig. Let us know what happens if you take some of this advice.
Low level Hum in Phono Signal Path
There is a distinct low level hum noise evident through my speakers when the phono mode is on. Can't be heard from the seating area but can be heard when your ear is in front of the speaker, either side. This noise is also dependent on the volume; the higher the volume the louder the noise. What is causing this noise and can it be remedied. (Equipment involved: BAT PK 5 phono preamp, Transrotor turntable with Benz Micro Ruby 3H cartridge)