I’ve been using Rockbox on Fuze v2 for a couple of weeks and haven’t encountered this sound issue. I update regularly, sometimes building from svn and sometimes downloading a daily build. At the moment I’m using yesterday’s daily build r26274. The sound is great, at least the equal of the original firmware imo. I’ve also been using Rockbox on a couple of old iRiver players, H140 & H340, for a few years. I recall when the H140 got an iRiver port there were similar anxieties/assertions/questions about the sound quality. Usually these are in terms that are hard to quantify and more importantly entirely subjective. It’s impossible to do a reasonable comparison by booting from one firmware to the other. For one thing the default volume levels are different and they must be identical in order to make any meaningful comparison (however sophisticated we like to think we are, inevitably in blind tests a small increase in volume leads to people identifying the exact same player as a different “better” player). Perhaps the only way to actually make a decent comparison is to use the headphone socket as a line out and capture to hard disk the same audio from each firmware at measured identical levels, and then to blind test it i.e. abx on a PC. Unless that kind of objective test is demonstrated, or there is some huge glaring error like distortion or clipping, then it’s pretty reasonable to assume that these kinds of anxieties do indeed arise from placebo or psychological factors, not from any actual, measurable or demonstrable difference.
It’s also worth checking your settings. For example is replay gain enabled on one but not the other? Is the pre-amp level the same on both? Is one firmware set to album gain and the other to track gain?