I've only heard the Adcom GFA-555 and agree with much of what various people have said about it here: http://www.stereophile.com/solidpoweramps/678/index.html

I have heard the Parasound HCA-1500A and thought it a little mushy compared to the detail apparent in the Adcoms I've listened to. Could have been the different terminal output devices though (loudspeakers).

That said if you can you should listen to both because it is unlikely my ears and perception are like yours.

I've owned the Adcom 555 Series II for many years and can account to it's ability to play on both sides of the fence. One side being great SQ for the money and the other side being able to handle long term abuse for many hours on end with no sweat. I had mine hooked up to a pair of Klipsch Chorus II's that played at near 120db for 6 hour stright jam sessions almost every weekend for many years. It still worked like new when I turned it over to a family memeber who still uses it today.

I don't have any experience with Parasound. For some unknown reason I never really found much interest in their products.