I experianced a similar thing on my fender strat. loosing a lot of treble when turning the volume down + sound getting muddier and muddier, although this is common on all standard strats.
I came across a company in the US called griblin enineering who make custom wiring harnesses to their own design which feature a "treble retention" circuit which basically counteracts the treble loss when turning the volume down. So now when I back the volume off my guiar retains a sweet tone with no muddiness !
I've seen treble etention circuits advertised for humbucker type pickups too on the net, a few companies make custom wiring harnesses.
The wiring harness cost me $70 , and it had a few other none standard controls on it, it really transfomed my strat. and gives me loads more tones.