Well, it's not an XR45 mod, but today I finally got around to doing my battery mod to my XR-25. Very simple- just wired in a 12V SLA battery into the main H-Bridge supply leads. The receiver still needs to be plugged in so that the low-level circuits have power, but the output stage is completely run from the battery. Power output should be 8-10 W in this arrangement. Volume control continues to work as expected as long as you're below about -20 or so on the dial.
I haven't put it in my main system yet, but it sounds pretty good on the RatShack Linaeum speakers in my computer room.