Okay, I'm not going into any details about the overall sound quality of the Redbook-only PCDPs currently in my collection. But I can definitely tell that both of my Sony players sound darker and have rolled-off highs compared to my Panasonics - both from the headphone output and the line output jacks. The Panasonics may be closer to 'neutral' than the Sonys. However, I've noticed some high-frequency distortion from my Panasonics with the anti-skip circuitry turned on (at worst, the sound quality is similar to that of a 128kbps MP3), due to the fact that the 40- or 48-second anti-skip is compressed. The SL-CT790's anti-skip circuitry cannot be turned off at all, and the North American market versions of this PCDP has no line output jack or optical output jack whatsoever - but even so, I prefer its (the permanently fudged SL-CT790's) sound quality to that of the other (fudged) Panasonics with their anti-skip circuitry turned on.
And as much as I thought I had loved my Panasonic SL-SW850, I found it somewhat lacking in overall performance compared to the other Panasonics in my collection: Its line output is grainy; its headphone output, somewhat echoey. But then again, it was a high-priced model that uses the innards of a much cheaper model.
As for Sony's G-Protection, current versions offer a choice between 'uncompressed' and 'compressed' buffering - but even uncompressed buffering cannot hide the fact that the DACs and op-amps inside post-Y2K Sony CD Walkman players are of inferior quality to those inside post-Y2K Panasonic PCDPs as a whole.
And no brief comparison is complete without a mention of how well the USA-market Panasonic SL-CT790's headphone output functions as a line output. Well, it actually works pretty well in that capacity, but its maximum settable volume is lower than the line output jacks on the other PCDPs.