While I no longer have my HD600's, I can't address your issue of the stock PB2 sounding too harsh and bright, but that is certainly not what I would call the native sound of the PB2. Given a proper source and used with balanced phones, it offers quite a well-balanced sound across the entire audio spectrum.
Rolling opamps will not make major changes in the SQ of the amp, as the differences between opamps (that are appropriate for use in a given amp) are subtle and the differences are more apparent in soundstaging and imaging. If you want a warmer sound, I would suggest that use of EQ is a more appropriate way of achieving the sound you want.
The PB2 is designed for balanced use and will deliver the best sound using the balanced outputs. The stock SQ is more neutral than warm and again, opamp rolling results in changes more appropriately called "tweaks" rather than major changes in SQ.
Harsh strident highs are more likely the result of a poor source somewhere upstream of the PB2, as the HD600 is not a headphone that typically exhibits this type of SQ nor is the PB2.
While the P4 offers more options for customizing by having socketed L/R, buffers, and ground, changing components will not make night and day changes in the SQ of the amp. The stock SQ of both the PB2 and P4 are more similar than different, with both having excellent tonal accuracy and balance as well as enough output power to drive any dynamic headphone to headbanging levels.
IMO, the P4 sounds better than the PB2 when using non-balanced phones from the SE output of the PB2.
Edited by HiFlight - 1/8/12 at 3:45pm