I've searched all over trying to figure out what frequency ATRAC's low pass is on a Long Play mini disc vs the normal compression. I have several sets of live shows that claim to be from a MD source but have a cutoff around 13khz. Sorry if this has been discussed, but I could not find anything. Thanks!

According to the following sources, LP4 mode uses a 13.5 kHz lowpass

http://en.wikipedia.org/wiki/ATRAC
http://www.answers.com/topic/atrac
http://psychcentral.com/psypsych/ATRAC

Thank you! I should have thought to check at wikipedia.

From my past experience with MD (Type-R/ATRAC3):

SP: 22.05kHz (i.e., no lowpass when dubbed via TOSLINK).
LP2: 17.5kHz
LP4: 13.5kHz.

Sounds like your recordings were done with LP4 (66kbps), which is a strange bitrate for concerts!