What makes this probably worse than usual is that the kind of noise artefacts produced by bad digital filters can easily mislead lossy psycho-acoustically modelled encoders to allocate spectral buckets to these artefacts rather than "real" content.
something innocuous like the steepness of the cutoff
Not all low pass filters are created equally, especially when the order of a "perfect" filter exceeds a few thousand, as few resamplers will bother with that.To be sure you're not suffering from resampler implementation bias, you should do manual resampling with SSRC in "high quality" mode using the standalone tool written by SHIBATA Naoki.Outside of proprietary closed-source resamplers, I'd assume anything less than Shibata's is broken. 44.1->48/32/24 resampling requires a relatively complex filter to do it "perfectly". (One of the reasons why 44.1KHz was chosen even though more "harmonic" values were known or even proposed at the time was precisely to make this kind of resampling "hard" for consumers.) The resampling ratio is a beast, in any event: 160:147 (for 44.1->48). EEs/DSP jocks will immediately understand the high order filter needed to do this job perfectly.
I looked at this once upon a time with simple test signals. The reconstructed wav files matched the originals well when there was no LAME resampling. Lame resamled signals showed a quite visible amount of aliasing, of the sort shown athttp://src.infinitewave.ca/by some of the less able resamplers.