I think the GFA-555 was designed by Nelson Pass (not the later versions of the 555). So were the GFA-565 mono monsters. And according to a test report in Stereophile ?? so was the GFP-565. Both the preamp and power amp 565's have issues with leaky (literally electrolytic leaking out of the capacitors) . The bad capacitors didn't show up until many years down the road (Looong out of warranty) -but with the power amps the fluid from the caps can short stuff out and result in pretty impressive failure. AFAIK - all Adcom equipment from that era needs to be recapped. I've got a GFP-565 here that also hisses - but pulled the covers and it has many obviously leaky capacitors. One of these dayz -I'll get around to recapping it and try it again - it's obviosly not working right as it sits .