I don't know if it's truly related to "bass control and speed," but I've seen a number of people (myself included) comment that the XA amps can be a little soft on bass. Like Peterayer, that can lead some people to prefer the X series over the XAs.
What kind of speakers do you have, and what kind of load do they present to the amps? That info might help guide people's responses here...
What kind of speakers do you have, and what kind of load do they present to the amps? That info might help guide people's responses here...