Hi, Nik:
Thanks for the list of your components. IMO, I think the "problem" you describe may not, infact, be a "problem" since your system is doing what it should. Let me explain.
As it turns out, you and I have some similar components in our system (my system is listed -- see link below my name). The Bryston amp and the Alpha-Core speaker cables are the common factor between our systems, and based on that (and listening that I have done to several Sim Audio components), I think that our systems both provide a fairly accurate picture of the recordings being played. Unfortunately, I have no direct experience with your speakers, which obviously constitute the greatest single variable in yours (or anyone's) system.
If you try to achieve a "warmer, wetter" sound (we'll leave the obvious allusions alone...) by changing cables, I think you will simply change the entire presentation of your system, and the qualities you currently like (and want to preserve) will be altered.
Alpha-Core Python cables are essentially a twisted configuration of the MI2 cables, which I am currently using. I have found this cable to have all of the properties you want -- excellent bass, and balanced mids and highs. Further, the Bryston 3B-ST is essentially neutral, and is good enough to reveal the transparency and detail provided by your Sim Audio preamp (much as my Bryston SP-1 does). Sim Audio and Bryston preamps have very similar tonal qualities, and I think both fall in the neutral to very-slightly-warm range. This leads me to think that the high-frequency "edginess" you are hearing on some recordings is because your system is accurate and is passing along the audio signal with genuine fidelity -- exactly what you want your system to do.
All good high-end systems involve compromises, and they come with a "good news" and "bad news" feature: they reveal both the best and the worst on recordings. If the system simply made all recordings "pleasant" (i.e., homogenized), then the system is not accurate.
There are several things you could try that may or may not yield the result you are seeking: try a different interconnect such as the excellent Alpha-Core Goertz TQ2's or Kimber Hero's, and you might try them with RCA rather than XLR jacks (in some systems, RCA terminations may sound a bit more "laid back" than XLR's).
I realize that my comments may not be what you wanted to hear, but I reiterate that if you essentially like the sound of your system EXCEPT ON SOME RECORDINGS, then the issue is really with the recordings, not your system. Assuming that you intend to keep your current speakers, there are only two "sure fire" solutions that I can think of to solve the problem you describe:
1. don't play the recordings that bother you (something I began doing some years ago);
OR
2. get a preamp that has tone controls that allow you to roll off the high frequencies on those recordings that bother you.
Hope you get some other responses that will provide advice that may not have occurred to me.
Best regards, and good listening.
