When you say "4 point option", I guess you you mean you have the "SRC quality" slider at the lowest setting? When resampling is required, the lower quality settings will use fewer points/samples (the lowest is indeed 4) and have more aliasing than the higher settings. In this case, it sounds like you like the sound of the aliasing when upsampling? So you want the "Apply sample rate to all file formats" option to only apply when the original sample rate is lower than the "Sample rate" setting?
Almost that, but with a user definable frequency value. I've uploaded some samples to the FTP, to illustrate this, the things I talk about are more noticable with headphones.
(I have my output set to 16-bit, 48 kHz, if that helps)
16 A Supplementary Lecture.at3 - 44kHz stream. Sounds good without resampling, sounds good with resampling with SRC quality set to the highest value, doesn't sound that good with SRC quality set to the lowest value (noticable ringing/aliasing).
BGMA05.ADX - low quality, 11300 Hz stream. Sounds muddy without resampling, sound muddy with resampling with SRC quality high, but surprisingly enough sounds really good with resampling and SRC quality low (any other option sounds muffled, the last one could seriously be mistaken for a 32 kHz one).
Ideally, I'd want an option that will handle this cases automatically - so that I could set the resample SRC quality to low, add a frequency value below which XMPlay will resample stuff (say, any output with frequency lower than 25 kHz), but leave better quality streams alone.
Let me know if you need further samples.