So, I checked the MX-110 Z-series service manual and there is no spec for output impedance called out. From looking at the schematic of the 6U8 circuitry, its not going to be easy to figure it out, either.
Here's my take and, perhaps, someone else can confirm or 'put me in my place' about it:
The final output stage, 1/2 of the 6U8, is configured as a cathode follower with about 22K used for the cathode resistor, which in normal circumstances, would could be used to approximate the output impedance of that stage. The problem here is that both the bass/treble tone circuit is included in the feedback network as is a feedback loop to the other 1/2 of the same 6U8. For the result of this, I'll assume these have no effect on the output impedance but that's just a cheap way of not doing a detailed analysis ;-).
Next, the last stage of amplification then drives the Balance and Volume controls before the audio is presented to the RCA jacks for Output. Since we have a 60K Balance control and a 40K Volume control wire in parallel, they are presenting their load to the ~22K output impedance of the final cathode follower stage. This effectively puts them in parallel to the 22K. So, doing the math for 3 resistors wired in parallel, we a result that _should_ come in lower than the typical 10K output impedance. Right?
