The only significant difference between standard MultEQ and the XT version is that the XT has higher resolution filters for correcting room response of your main speakers. Both apply the same high res correction for the sub.
Either of the two flavors of MultEQ will greatly benefit your system if you have a subwoofer due to the resolution of filters it applies in smoothing out your subwoofer's in-room response. I have heard very high end setups that still benefitted from bass room correction, i was surprised the first time, and turned me into a believer in DSP hehe. There is much more involved here than EQ.