I am a new owner of a used EAD Powermaster 1000...first of, I truly love this amp. I originally had a Parasound HCA 1205A and heard the difference as soon as I turned the PM 1000 on. However, someone told me about a "unity gain upgrade" for the amp...or something like that. Can someone explain to me what is this upgrade? I also heard that the PM 1000 desing was revised to improve audio performance and that all amps shipped since Oct 1, 2001 included the enhancement. I'm not sure if mine includes the enhancement. Can one tell by the serial number or a change in the faceplate? Thanks in advance.