I have a number of 16-bit/48kHz 5.1 files I'm encoding with Nero AAC 1.0.7.0. When using -br 450000, the encoding goes fine with an average bit rate of 469 kb/s. But when I use -br 450000 -2pass, the average bit rate drops to 406 kb/s, and the sound quality drops severely. I can't ABX the one-pass encode, but the two-pass encode sounds like a poor MP3 stream circa 1999.

Surely using 2-pass should result in a better encoding?

I can upload short samples for anyone who doesn't have 5.1 audio lying around.

I will check this when I get back to work next week.
You could try version 1.1.34.0 (http://www.listening-tests.info/mf-64-1/NeroDigitalAudio.zip) and see if the same thing happens.

I tried the new encoder, but the issue is still present.

I've uploaded (30 second? legal?) clips demonstrating it:

http://download.yousendit.com/1C23A3F32684B36E

Ok, found the problem...
Better not use -2pass on 5.1 files with current encoder versions.

I have the same problem! -2pass is not stable

The same problem with 5.1 or also with other configurations?

I didn't have a problem with stereo/2-pass so far.