Does the sound fatten up even if you only back either volume control off just a little bit, like by 10% or so? If so, this is normal for vintage Gibsons. With both volume controls at maximum the two pickups are wired directly in parallel and each applies a fairly high load to the other. As soon as you back either volume control off even just a little you throw 100k or so of resistance between the two pickups which isolates them so they no longer load each other. The 100k of resistance isn't that significant to the high impedance amp, so you get almost full volume from both pickups and the sound is fat.
A "cure" for this is to put a 47k resistor between each volume pot and the pickup switch. This amount of resistance will barely reduce your volume at all, but there will always be at least 94k of resistance between the two pickups, preventing them from loading each other.
Not all vintage gibsons do this because often the cheap pots they used didn't go all the way to the end of their electrical travel anyway, leaving a little resistance between the pickups even at full volume.
I'm glad you brought this up, I need to make mention of this "fix" on the website because I've not seen anyone else note it.
Finally, I should add that a similar dynamic is at work in Fender style guitars. When you select the neck and middle pickups together, each is loading the other, likewise when you choose the mid and bridge pickups together (or the neck and bridge on a Tele style guitar). That is why the signal usually gets a little quieter, instead of louder, when you select the combination positions on the selector switch. If you want a different sound from your Strat or Tele try putting about a 20k resistor between the hot lead of each pickup and the selector switch. You'll sacrifice a bit of overall volume (not very noticeable at all if you're feeding a hi impedance amp or effect) but will get a fatter sound in the mixed positions.