The Heresy IV and LaScala AL5 are easy to drive speakers with a high sensitivity of 99dB and 105dB respectively, and a nominal impedance of 8 ohms. With these speakers it is not a surprise the Pass Labs INT-25 sounded best in comparison to all other higher spec amps which include the INT-60 and 250. Imagine using flea-powered tube amps with the Klipsch horn speakers. With these high sensitivity horns, other high powered amps will have the potential of sounding aggressive.
Move to more conventional harder to drive speakers with 86 to 88dB and lowish impedance of say 4 to 6 ohms perhaps dropping down to 3 ohms minimum, the tables will likely be turned as the INT-25 takes a back seat while the larger amps which include Luxman L-590AXII sounding better with these select speakers. Think Dynaudio, B&W, Marten etc. as these speakers will have the tendency of sounding flat and lifeless if they don’t get enough current and drive from the amp.
Move to more conventional harder to drive speakers with 86 to 88dB and lowish impedance of say 4 to 6 ohms perhaps dropping down to 3 ohms minimum, the tables will likely be turned as the INT-25 takes a back seat while the larger amps which include Luxman L-590AXII sounding better with these select speakers. Think Dynaudio, B&W, Marten etc. as these speakers will have the tendency of sounding flat and lifeless if they don’t get enough current and drive from the amp.