It's very possible that the DC blocking filter in the converters is tracking the sample rate. I.e. as you go up in sample rate, the DC blocking filter's cutoff rises (1 Hz at 48k, 2 Hz at 96k, and 4 Hz at 192k).
The increasing phase shift from the DC blocking filter smears the low-end, and causes low-end transients (like kick) to loose their punch and transparancy. To me, it sounds like a window sliding up the spectrum, you loose low-end as you gain high-end.
This isn't a slam on high sampling rates, because there are some good converters that do this correctly, and these *do* sound better at higher rates despite technical arguments to the contrary (ears are the reference, not theory).
Also, in DSP systems, higher sampling rates do have advantages as well as disadvantages (higher processing load, more disk space/fewer tracks, etc.). Specifically, well designed DSP filters will sound better at higher sampling rates because their responses can more closely approach the ideal analog response (in technical terms, the s-plane to Z-plane warping is more linear). Conversely, poorly designed filters may sound worse, especially at low frequencies because of data precision issues.
Futhermore, at higher sampling rates, analog anti-aliasing and anti-imaging filter requirements are less stringent (allowing less phase shift in the main audio band), and system latency can be reduced so digital processors can be used in real-time monitoring situations (especially vocal tracking).
-Joe
Joe Bryan
VP Engineering
Universal Audio