Threshold S-500 Series 2 Statis stereo power amplifier . 250 watts per channel into 8 ohm's, 500 watts per channel into 4 ohm's. The S-500 can also drive a 2 ohm load ! The S-500 was designed by Nelson Pass to handle difficult loads .( http://www.vintageamprepair.net/s500.html ) . I purchased this S-500 from the original owner in 2016 , but I never used it because I down sized my system . The original owner had switchable Balanced inputs installed , at a cost of $700.00 . You can run single ended as well as balanced interconnects and switch between the two ( power amp turned off !) .