I agree that a 500k pot instead of the 300k that's in there now should open it up some more. You could also try a 1Meg pot. That was popular in the 80s, but seems to have fallen out of style. Either option should give you a touch more output, too.
You could try changing the tone cap, too. They're pretty cheap (<$1), so you could get a few to experiment with. 0.022uf is the typical value for a bridge HB. Changing the value really only changes how much high end is rolled off by the tone control. So, I usually just stick with that value, although you could try something like an 0.015uf (typical neck HB value), which should allow a little more high end through when the tone pot is all the way up.
Changing what the caps are made of, to me, seems to have a greater impact on tone. Ceramic, polyester, polypropylene, and paper-in-oil are some common choices. PIO seems to be pretty popular these days, but I usually prefer polyester. To my ears, I hear a lot more clarity (not necessarily high end, but just more clear note definition). Some of these (like reissues of specific capacitors) you can spend a lot more money on, but they all work the same way. I usually use Orange Drops or Mallories. Solen Fast Caps are good, too, but you're getting into audiophile quality and prices.
If you use your volume pot a lot and are losing high end when rolling it down, you could try a treble bleed. Basically, this is a resistor and small cap in parallel with the volume pot that lets more high end through when turning the volume down. Most of my guitars don't need it, but a few do benefit.
I'm guessing you've tried removing the covers (if they have covers on them) and adjusting the screw poles either higher or lower. Even though I think the 498T uses symmetrical coils, you can still effect the bass/treble balance by raising or lowering the screws in relation to the slugs. Another thought is either lowering or raising the entire pickup. Lower usually opens things up a little, but it's a trade off as it lowers the overall output, too. There's usually a sweet spot in there that takes a little while to find.
Anyway, just a couple of thoughts on things I've done to tweak a pup. Of course, and as always, YMMV.
