Original NE5532 for Beresford 7520?

I looked on ebay, there was "NE5532, NE5532N, NE5532P, NE5532AP etc", any difference and which is originally used?

Also, some said the opamp used in fixed/variable affects the sound in headphone out, is the reverse true?

Thanks

They are NE5532P, IIRC. There are two opamps used in that unit, one is for the line output and the other is for the headphones output.