Increased the amount of cotton I put in the HOK cups. Previously, I had just enough to cover the back of the cup. Now, I put in enough to fill the entire cup even when compressed. Not much change in sound.
The biggest issue with the HOK, one I've had since day one, is that the driver puts out a massive treble spike at 9 kHz or so. It seemingly remains unmolested by felt, nylon, and wool discs placed in front of the driver, regardless of their quantity. The only efficient mod for it seems EQ.