Difference between NAD 2400PE & 2400 THX

I can't say with absolute certainty that the two amps are identical, but I can tell you that the "THX" appelation is largely meaningless. The right to use the "THX" trademark and logo on a piece of audio equipment is subject to two factors (neither of which has much to do with insuring "better" audio quality):

1. "THX" publishes a set of performance standards that THX-licensed equipment must meet. These standards were devised originally with movie theaters in mind. With HT equipment, the standards pertain primarily to sustained and peak power output, and distortion levels, plus adherence to some other performance criteria that the "THX" folks think are relevant to good HT sound. In point of fact, almost all better quality audiophile amps meet or exceed these standards. Many people assume that the "THX" trademark and logo is a guarantee of better performance, which "ain't necessarily so". In short, for better quality amps, the "THX" logo is mainly a marketing gimmick.

2. The REAL meaning of the "THX" trademark and logo is that the manufacturer has paid a licensing fee to use the logo, which is assumed to be a strong selling feature to HT enthusiasts.

As far as I know, there are no significant differences between the two NAD 2400 amps, except in how the power output is rated. For practical purposes, it's a non-issue.

I'm not sure about the THX, but the PE stood for "power envelope." The NAD's in this series had 6db headroom, or in other terms, pretty high current for a given watts into 8 Ohms rating.