most likely the wiring for the treble setting(bridge pickup) is wrong or problem with the volume pot(treble/bridge pickup).
for the first case, when pickup selector at treble, vol pot at max and still soft wheras selector switch at rhythm, vol is normal, thats a sign that theres some problem with the bridge pickup wiring or vol pot.
when in the middle position, depending on how the wiring is done at factory(or by original owner if its a 2nd hand instrument), theres 2 type of wiring for a 2 humbucker, 2 vol and 2 tone configuration
1) in middle position, turning either volume pot/tone pot, will affect both bridge(treble) and neck(rhythm) pickup
2) in middle position, each set of volume and tone pot will only control individual pickup. Thats to say, if you turn down the bridge pickup volume/tone pot, only vol/tone for bridge pickup will be affected and not affect the neck pickup vol/tone at all. The same thing apply when neck pickup is concern, only neck pickup vol/tone is affected and not bridge pickup
from the description, when in middle position, when both volume at max, you cant hear a thing, but by turning one down, you can hear it. The thing here would be, which vol control did you turn down.
to know which pickup is selected by the selector switch and its own volume/tone pot control, with the geetar connected to amp thats "on" use a small screwdriver or metallic thingy, gently tap on the pickup, you shud be able to hear the "thud" sound being amplified.
heh, if the above sounds too much, bring the guitar to geetar tech to take a look, they should be able to help you.
