I have a 1989 610 and a newer 330 both with Hi Gains in them.. The pickups sound amazing when full but when i turn down the volume knob on either guitar it's just muddy and doesn't clean up well. I actually even rewired the 610 with CTS 330k pots and no difference. On my Les Paul, Duesy etc this isn't the case. Is this a symptom of the Hi Gains? Is there something i can do to change this?

I can't speak to the 610, but I have a 360 and I simply don't have this issue. I wonder if you could give us a few more variables to understand the situation. The most important one I think is learning how your tone knobs are set, including the fifth knob. Also, can you tell us amps, pedals, and situation (live / bedroom)?

I think most guitars sound kinda muddy when you turn their onboard volume down. Try a Compressor.

AGNP is right. All passive instruments load down rapidly past a certain point on the volume pots resulting in a loss of high end. If your amp input is low impedance it will be even worse. Try a DI pedal in your signal chain to buffer the impedance.

The only reason to use your guitar's volume controls instead of the switch is if you are turning one pickup down relative to the other to create a subtle tonal effect. Otherwise use the switch to go between pickups and the amp for your volume. Only the top 10% of the volume pot's range tends to be useable.

This can depend on the taper of the pot - linear taper v. audio taper. Audio taper is pretty standard, and supposedly adjusts volume according to the way the human ear hears it to be more gradual, but not all agree on that. Since it can be true that only the top part of the range is audible, some just use that part of their knob's range (I think it's a bit more than 10% though - that would mean your knobs are usable only on 9 or 10). IOW, they think of 5 or 6 as the lowest setting, and anything below that as being equivalent to zero/totally off. That can make for more effective volume swells on certain instruments - without having to dial through the full rotation of the knob - kind of N/A on Ricks, though, because of the volume knob placement.

It's down to what's known as "transformer effect" The higher the impedence across a pickup, the greater its high frequence response. when you turn the pot down, you're lowering the impedance across the pickup, and it sounds duller.
If you find this really irritating, there is a simple mod you can have done. I did this to my telecaster to keep it bright when the volume pots are down. Parts are no more than a few bucks.

See
http://www.guitarelectronics.com/produc ... istor.html