Too Much Gain the Issue ? (preamp and amp)

24dB is an awful lot of gain for a preamp. 26dB is sort of a surprisingly high gain for a 30 Watt SS power amp. Yeah, that might not be the most optimal match - but the volume attenuator on the preamp should keep the amp's input stage from getting overloaded and causing the sort of sonic issues you describe. So as @erik_squires mentioned, your main artifact of too much gain should just be noise floor. 

The Cary is tubed - LOTS of 6SN7 - so you can tube roll to try and mitigate some of that sibilance. If you have Electro Harmonix 6SN7 in there now, then starting to replace them with any decent vintage tube should help a lot. 

In modern era, preamps with gains over 20dB (almost always tube based 6SN7s or 12AX7s) are sort of an awkward fit. 

The input impedance of the Pass is too high for the SLP-05.

10K ohm is recommended (per a Stereophile review/measurements) and the Pass is 50K/100K depending upon single ended/balanced connection.

The mismatch will theoretically give a leaner sound (less bass).

DeKay

@avanti1960 I experience zero brightness, harshness or any type of sibilance issues with my XA30.8. I pair it with a Pass XP-12 preamp (9.3db total gain) using XLR cables.

Could be a combination of mismatched impedance exacerbated by too much gain. 

@Dekay

Based on the link below, you misread.  I quoteth:

The Cary preamp needs to be used with power amplifiers having input impedances of 10k ohms or greater if the bass is not to sound a little lean.

As you point out, the input impedance of 50k Ohms listed on the Pass site is more than enough.

I am so not a Pass fan, but what I have read from those I respect is that  these amps are very sensitive to speakers, and with the right speakers will sound full range.

I will say that when I repped InnerSound (ancient history) I took speakers to a dealer and for whatever reason the Pass with those transmission line speakers could not produce any bass at all.