Gary,
I found that the X.5 really benefited from an extended warm-up time. It took at least 8 hours for it to become musical and listenable for me. At 12+ hours, it was even better. During the winter, I would leave them on 24/7, and they sounded good. During the summer, forget it. However, the XA.5 sounds better cold than the X.5 after 24 hours.
For the XA.5, I think there is some small improvement after 2-3 hours. There may be additional subtle improvements after 8-12 hours, but these are quite minor. If someone was coming over for a listen, I'd leave the amps on overnight to get the best sound, to be sure.
For bass control, I think this can be optimized by component and cable matching. But the X.5 would probably have more "fast" bass.