KDX Runs Poorly at WOT at Low Engine Speeds

I have a 1992 KDX 200. The bike runs great at WOT at high engine speeds. Thus I assume that the main jet size is very close to perfect (the spark plug confirms this.) However, the bike runs poorly at WOT at low engine speeds, when vacuum is low. It used to run fine, then somewhat suddenly started pooping out, which points to something suddenly happening.

It runs poorly as soon as the bike is started which indicates it isn't necessarily a heat related problem. The problem might indicate a radical error in fuel/air ratio under these conditions: air leak, bad reeds, float too high or too low. Reeds would seem to be ruled out since I replaced them recently with a new set from the dealer.

I recently disassembled the carb and soaked it overnight. The next day I cleaned it with new gasoline and used carb cleaner to make sure all of the orfices/lines weren't plugged. I also checked and adjusted the float level slightly.

I am running non-stock expansion chamber, silencer, air box(top) and reed valve.

In summary the tune-up items that I recently performed are the following:
new reeds
carb soaked and cleaned
new plug
repacked the silencer pipe
cleaned the air cleaner

Suggestions?

I need to explain what I mean when I say WOT at low engine speeds.
I meant to describe it as more of a bogging. For example, if I'm climbing a hill and the rpms are relatively low and I give it more gas, the bike will not respond (this is even the case when I'm in 1st gear). It feels like the carb is either getting too much fuel or too little. This situation has gotten worse and can even be experienced on relatively flat ground if the rpms are low and gas is cranked hard. If the rpms are high and the throttle is cranked, the bike will respond.
As far as the reeds are concerned I replaced them recently hoping that they would help cure the problem.
A "friend" last night suggested that I look at my power valve. He suggested that it might be opening too early. I will try to disable the power valve temporarily and see if that changes the symptons.

Thanks for the interesting points in the reply from Levert.

I will try to verify PV operation at this point. I think the Halmhotz Resonator chamber has an external cover (left side?) that could be removed and checked for blockage (which would render it useless) and might provide a view into the valve to check for sticking. There should be a hole in the PV from the HR into the main exhaust port at anything below 6000 RPM (including engine off.) Even if it is stuck in this position, I would think that the bottom power would be okay.

I will check for air leaks by getting a steady idle and spraying a fluid around the manifold joints.

I'm not sure of a good way to check main seals, but I would think you would get blow-by through the trans vent and/or disappearing tranny fluid if there was a leak here.

I'm not sure of a good way to check spark, but that would be something to try - maybe only visually.