Just one thought, 50k might not be low enought. I tried it with a ~200 Ohm output to a ~50k input. It didn't work. The music lost all sense of dynamics and just laid there. Might have been interconects.
I will try 10k soon.
Beter yet put your pot in your amp as the input(grid) resistor.
Bit of a pest for monoblocks though...John