I realize this is a relatively old thread I'm replying to, but I'm using MB version 3.5.8447 (most current version) on a Windows 10 OS on a Dell AIO desktop and I just noticed a similar issue of extremely worse sound quality in WMP than MB. It started with one song having noticeably muddled mid-ranges and well, everything but the vocals. I thought it was somehow a result of being a 128 bit rate file instead of .FLAC until I played another song back and forth in WMP and then MB and then more. This is not a minor difference. The total sound quality is night and day with the former sounding like a concert hall while MB sounds tinny, has muddled almost indistinguishable instruments, "flat" bass as when hitting the edge of a drum vs. the center, and lower overall volume. The vocals sound like they're coming through a washcloth in MB vs sounding crisp and full-bodied in WMP. I don't recall this issue being the case until a day or two ago. Differences: I did set the equalizer in MB, and have since tried resetting it, but it made no difference.