I have a Pass 250.8 on a pair of Wilson Sabrina in a neutral treated room. I run a Pass XP 10 pre amp and the system is neutral or possibly leans toward the warm side. I'd think that it'd be too warm if a warm pre amp was added into the system given the Pass amp. it may be more of a room treatment issue you have if you find it too bright- or your speakers ? Changing the pre amp out for 6k to possibly gain a slightly warmer sound may not be the best bang, nor the most predictable means of testing things out. Try the EQ settings in Roon if you use it. They're very flexible, you can set many different bands up with varying Q factors, even set pre set curves for low level, loud listening and much more. It's included with Roon.
