I recently spent about 1 week with my friend's slp-05. I thought is was very detailed, excellent bass, nice mids, but I could not help but notice it was "hot" sounding, ie tipped up in the trebles. I would not call it harsh, but it called attention to itself. Now, I have heard this same preamp in his system (we have identical speakers and very similiar digital front ends) and I have not noticed this with the treble. Instead, it simply sounds like a synergistic match to my ears...
I think it is clear that there may be a compatability issue with the SLP-05/Pass amps...
(also I have an LS15 on hand and it is a great match with the Pass. The tipped up treble is not really a trait with this match.)