Hi,
I haven't heard the INT-150, but I did upgrade from the X250.5 to the XA-100.5 monos. I agree with Peter. At least in my case, the mono amplifiers provided nice improvements in imaging and soundstage depth over the stereo amplifier. The XA series is smoother, warmer, pulls out more low level detail and has better low end control compared to the X250.5.
If you find you do need a preamp for impedance matching purposes, but don't need extra gain and don't want to lay out the bucks for an XP-10, don't discount the Pass B1 buffer. It sounds quite good with the XA series amps, it is extremely quiet, although with a very lively, somewhat forward presentation.
Alan