With respect to replenished Xtol, did you follow the times for "seasoned" developer? I found with Xtol that it was affected the least by speed change of any replenished developer I used (HC110, Microdol-X, D-76, TMax RS, Ilfotec DD, Ethol UFG, Sprint) and I checked very carefully. Increasing the time works with replenished Xtol, and without giving excessive contrast.
The low volume you used, to me, suggests your developer was always beginning to fail, rather than that the speed loss was due to replenishment.
Leica M7, 50mm Summicron @ F4 APX 400 @ EI400, replenished Xtol, 10:30 at 20C. Agitation for 1 min, then 10s every 30s by nitrogen gas burst in a dip-and-dunk system. The neg has plenty of detail in the suit, I just couldn't capture it with my -then flatbed scanner. The neg has a CI of 0.585 and a toe density that is spot-on. But the tank had at least 50 rolls a week go through it. The system was 15L and had not been re-started for 3 years when this film was developed in December 2004. I also found that replenished Xtol needed more agitation than I used with manual development.