I haven't heard all of these units. I have an XP-12 that I use with a Pass Labs XA-25 and a First Watt J2. The XP-12 is a great pre amp, but if I had to choose between your two options, I'd choose the INT-60, since it runs deeper in Class A than the X150.8. Of course, this assumes you don't need the extra headroom in the X150.8.
If you haven't already, I'd reach out to both Pass Labs and Mark at Reno Hi-Fi. Talk to them about your sonic priorities. They are all friendly, responsive, and helpful.
If you haven't already, I'd reach out to both Pass Labs and Mark at Reno Hi-Fi. Talk to them about your sonic priorities. They are all friendly, responsive, and helpful.