What S/N ratio is the Meridian config program telling you that you are getting?
Have you tried to listen to the output to see if there is a lot of hum in the system? What happens if you move the mic closer to the speakers?
Can you borrow a Tandy SPL meter to try?
Rgds
Stuart

Linn LP12, Ittok LV11, Arkiv, Linn Kairn as a phono stage, MPmax 221, DSP6000's, WiiM Pro streamer.