Tone is the issue. The HB sounds about right at full volume, but gets "thinner" as volume is turned down.I don't think that the value of the pot really matters when your volume is turned up to 10. A 500 or 250 is going to read about 0 ohms. It might affect your tone but not volume. I would check to see if all wiring is intact and correct.
So here's what happened: I replaced the cranky tone pot with a 500Kohm audio taper and replaced the three position switch with a 250Kohm blend pot. Also raised the HB up and dropped the P90 a hair (didn't want to lose the raunch!). The blend lets me go from almost full HB with a taste of P90 to the other extreme and there are an almost infinite # of stops along the way and some of them, for reasons yet unknown are really neat. The different pick ups respond slightly differently to changes in the vol and tone depending on where the blend is set, and about half way beteween both and HB there is a spot where it almost sounds out of phase, then rolling thru brings up the classic HB sound; going the other way, there is a monster spot about two thirds of the way between both and P90 where the thing just roars, then settles down into a classic P90 grind. Hitting a chord or note and rolling the blend is really neat, almost like a variable chorus.
I'm thinking the treble bleed might be fun to try. This guitar is nuts now.