Too Much Gain the Issue ? (preamp and amp)

sibilant in the upper midrange - several issues can cause this

1-gain

2-bias adjustment

3-source

Happy Listening.

Here's a USA purchase source for Rothwell, but I'm not familiar with them (purchased mine almost 20 years ago).

They are listed under accessories and I think that I paid $40/pair ages ago.

I went for less expensive as the AI preamp was a loaner from a friend.

DeKay

I use Rothwell 10db XLR attenuators and they work wonders between my tube pre and power amp..  I bought them new from Analogue Seduction on EBay and received them from the UK in less than a week.  

The Cary is already producing the gain; attenuators can help push noise floor hiss (NOT hissy treble) down and enable more volume control range but OP didn't complain about those issues, and they likely won't do anything to address the reported sibilance. Getting a super-low output MC to force a "need" for the high gain is also unlikely to resolve the issue here.

As OP reports clean smooth sound with XA25, he probably already has his answer - the Cary & X30.8 might just be a bad match. 

@stevehuff is running Cary SLP-05 with XA60.8 amps  

@avanti1960 may be worth reaching out to see if Steve observed something similar to what you’re running into.