The Pass INT40 is a better amp for the HD800/800S. The HPA is just a cut down tube speaker amp and overpriced. Might as well go for the real deal and run them off taps. Re: tubes, the Allnic HPA5000 works quite well with the 800. Two SS amps that I personally prefer with the HD800S are the Rudistor RP030 and the Liquid Gold.