10-20 % difference in the X2.5 compared to the X-1. There already was a thread on this a few weeks back. It also depends on the rest of your system. The Pass is very natural and neutral sounding so you will not get a tube mid range sound. They are very coherent top to bottom not too forward not to laid back. Never bright or harsh. A tube preamp may give you more texture in the mids but will not do the bass as the Pass will and some are not as extended in the highs.
Happy Listening.
Happy Listening.