I agree, it would be nice to have some indication as to the quality of the resampler used.
The only relevant indication on that (to me) would be reproducible blind tests.
And that seems to be a very big problem when it concerns up-sampling.
Even a sensible notion such as that up-sampling would best be done in factors of natural numbers is sometimes disputed by some people with solid technical backgrounds who say it really doesn't make any discernible difference.
So my advice to anyone that has some feelings of insecurity about the matter of up-sampling (in relation to listening and enjoying music, which is what any music player such as MusicBee is intended for):
1. Either don't use up-sampling. (you can then be confident that MusicBee will output a bit-perfect signal)
2. Or use your ears, and decide for yourself if you are able to hear differences, and then decide on the best approach for you and your equipment and circumstances. And that will be strictly personal.
To the respected forum members participating in this thread:
To test various upsamplers yourself, you don't need to wait for further explanations about the way how MusicBee uses upsampling, nor what upsampling plugins do work, or which ones don't.
There are plenty ways to try out different up-sampling methods and algorithms, without any need to (solely) use MusicBee for testing them.