This is actually a common situation for people who handle a lot of gear. As a reviewer I run into such things on a continual basis. I had all kinds of interesting outcomes with the Kinki Studio EX-M1+ integrated that can operate also in dedicated preamp or amp modes. Wildly different outcomes are possible depending upon which gear, cabling, etc. are used. So, don't be shocked or horrified at the result.
I am of the camp that would say the preamp of the Peachtree was designed particularly for use with the internal Class D, and it should not be expected that it would excel with any given other amp.
The fact is, no one knows the outcome of a combo of gear until it is tried. I have often had unusual combinations of gear sound better/worse than any other possible combination/component. The performance spectrum and personal preference spectrum together is VERY wide.
There are hundreds of outcomes when mixing gear, not to mention cabling, and it is very likely that the proper preamp with the Pass amp will bring that combo to another new threshold of sound quality. I would most definitely not blame the amp. This is why I build several systems when reviewing, because you don't know the innate character/capacity of the product unless you do.
Just the other day I put together a rig with some Pass Labs amplification on review (No, not saying which) and the Exogal Comet DAC ahead of it. Outcome, just as you describe; too warm and lacking to my satisfaction in definition. Not a great combo, although I have made stunning rigs with both of these items with very satisfactory sound.
Then, I went through an upgrade to the Small Green Computer sonicTransporter and SONOR Signature Rendu SE systemOptique that I reviewed and use for source. BOOM, now the same combo was rich, luxurious, no longer suffering the milky, indistinct sound. Had I tried it after the upgrades were done I would think the combo to be all that - and it is, given the "new" source.
Ergo, all components, etc. combine to yield the result. It's just not as simple as this or that component is not good, is good, etc.