Have you used that amp switcher before with your M33 ? It could possibly cause a problem if the switcher uses a common ground but I'm not sure. NAD warns against grounding any of the amp outputs.
M33 Manual Page 9 that I found on the internet:
IMPORTANT NOTES
- The blue terminals must never be connected to ground (earth).
- Never connect the blue terminals together or to any common ground device.
- Do not connect the output of this amplifier to any headphone adapter,
speaker switch or any device that uses common ground for left and
right channels.
If you use the amp switcher [speaker output switcher?] you might want to power-OFF both amps for several seconds before you switch then power-ON only the amp under test.
