Pass Labs X-1 -VS- Adcom GFP-750 Preamp

This thread is for people who own, or have owned both the Pass Labs X-1 and the Adcom GFP-750 preamps.
I am curious because the circuitry in both preamps was more or less designed by Nelson Pass.
What all sonic advantages does the Pass Labs X-1 have over the Adcom GFP-750, top to bottom and everything in between, when using the Adcom in both its Active mode and in its passive mode?
Does the Adcom have ANY sonic advantages at all over the Pass, or only disadvantages?
Is the Pass X-1's sonic improvement small, moderate, big, or dramatic?
Finally to those or have heard, owned, or compared both:
What all sonic improvements does the Pass Labs X-1 offer over the Pass Labs X-2.5 preamp?
How noticable are the improvements?
Thanks in advance.