Hum is practically normal with MC cartridges, if you don't try to ground the TT, TA, preamp combination. It takes some experimenting. In your otherwise very thorough description, you did not mention your grounding scheme, so I thought I'd give my 2cents worth.
Moving coil cartridge (AT OC-9) and Hum
I have 2 systems, the first uses an SME/Thorens with Supex MC cartridge into a Accuphase C-200 pre-amp. The second uses an OriginLive modified RB250/Revolver turntable with an AT OC-9 MC cartridge into a Carver CT3 preamp. Both systems use electronic cross-overs to feed multi-amp speaker systems. I have a Marshal Leach designed battery powered pre-preamp that when used on the first system produced no audible hum. Moving this pre-preamp to the second system however produced hum. The second pre-preamp is an older Dayton-Wright version that seems meticulously made and has a outboard remote power supply. It however produces hum on both systems albeit quite a bit less on the first system than on the second. The battery powered unit has much less hum on the second system than the Dayton Wright but the level is still unaccepatable. The hum is only audible while switched to phono on both systems. Is the OC-9 cartridge the culprit or is it the Carver CT-3 that makes the second system so prone to hum? I suppose I should switch turnatbles. The second system was only recently upgraded and with a MM cartridge, the hum was nonexistant, so I am very suspicious of the OC-9. Has anyone had experience with hum on MC cartridges (in particular the Audi-Technica OC-9) and can offer remedies?