Epifani PS600 bass amp. Notice the integrated switchmode supply and Class-D amp in the center. Very difficult to work on if the issue is in there. Lots of SMD and modular stuff. There’s about 1Vpp of ultrasonic noise (190kHz) on the output of these amps at all times — not sure whether this is a carrier residual from the Class-D power amp or SMPS noise. In most cases it’s a non-issue because speakers won’t reproduce it (and you couldn’t hear it if they did). Nice thing is that it does 550W into 4 ohms and only weighs about 10 pounds.