MakeShift
I've got a Gibson Classic 57+ in the bridge and a 490R in the neck of my Epi 335; both sound sweet. I don't recall having any issues with hum while recording. As Karyn has mentioned though, manny things can be the cause...dimmer switch, wall outlet, computer monitor, etc.

This I feel I really need to learn about the electric guitar, the qualities of the pick up positions. I like the idea of 2 different pickups, and changing my favored position first, so I can make a more educated choice about the second, or even if the first choice wasn't quite right and hearing it in the other position and upgrading the favoured again. Oh GAS

+1 to not screwing with a ground wire. It sounds like your environment is highly charged and not properly grounded in the first place, so creating a ground may just cause more issues (as you know the ground would be active full time and running current).
 
A few years ago I got on a pickup binge and redid my first guitar from scratch electrically. I happened upon this site and modified his design to only use 6 switches (no phase switch), and 4 of those were embedded into Seymour Duncan TS-1s Triple Shot Flat Pickup Mounting Rings (so I only added one switch to the face of the guitar). This is not a "quick-and-easy" project, and I modified his diagram to suit my needs to have no dead spots in the switching pattern and for the switches to be intuitive. That site has some nice guitar info on it in general in case you are interested in reading; but also realize that in your current environment that split coil settings (single coil taps) would cause a "hum nightmare" for you (so you would lose the versatility of the configuration anyway).
 
BTW, did you try out SPAN and gating/EQ with your hum issue?

It's didn't get a chance to try span, I doubt I will until after the weekend, as this is my busiest time. I had a little while to mes around with things last night, but in the end I just sat an worked on playing.

I did however pull the switch out, unscrewed the plates, to check for visible oxidisation (non), and lightly rub the contacts, check to see if the inner plates were deflecting (they are) when contact is made in all 3 positions. 
 
Playing last night I found that central position only seems to work if both volume knobs are on at least .5
 
Bridge pick up > 0.5 no sound from neck pick up
Bridge pick up > 0.5 neck pick up volume knob functions normally for 0.5-10
 
Neck pick up > 0.5 no sound from bridge pick up
Neck pick up > 0.5 bridge pick up volume knob functions normally for 0.5-10
 
 
 

Ah yes, the Gibson wiring quirk...  That's actually standard and expected behaviour on most Gibsons and any other guitar that's wired the same way.
 
You can blend both pickups in the mid position, but if you turn either right down to zero (or almost zero) it mutes the whole guitar.

Actually, that is normal, my 335 dives dramatically when in series and one pot reaches zero. I also have an Ibanez jazz guitar that kills the signal completely in that configuration. When in series, current flows through both pickups, so if one pot reaches infinity, current cannot flow through either.

Ahh ok. I don't think this was how it behaving last week, which seemed just the one pick up. So maybe after all the playing and dismantling I've made some progress to normal operation.