I've owned the Aleph 3, 5 and 2 and the XA160, XA100.5 and now the XA160.5. I have never owned the X or X.5 series. You can read my review of the XA100.5 on this site. I think the XA.5 are the best amps that Pass has designed.
Regarding your question about the first 60 watts: I replaced the XA160 with the XA100.5 and probably listened most of the time within the same 60 watts into 8 ohms on the same speakers because I never saw the needles move. However, the XA100.5 sounded much better and more powerful ie effortless. There was an ease to the sound. Other aspects also improved as described in the review. It was a huge improvement. My new speakers need even more power, so I moved up to the XA160.5. I can't comment on the effect of the number of stages. Hope that helps.
Regarding your question about the first 60 watts: I replaced the XA160 with the XA100.5 and probably listened most of the time within the same 60 watts into 8 ohms on the same speakers because I never saw the needles move. However, the XA100.5 sounded much better and more powerful ie effortless. There was an ease to the sound. Other aspects also improved as described in the review. It was a huge improvement. My new speakers need even more power, so I moved up to the XA160.5. I can't comment on the effect of the number of stages. Hope that helps.