I couldn't find anothe spot in the circuit to put a mono switch. Yes, it loads the cart down a bit more, an unfortunate drawback. But this is the best place to guarantee same signal to both channels.
Any other place would start to get dependent on exact tube section matching. Other places are possible, just didn't seem right to me. You could always sum the outputs via 1k resistors and try it that way. I was guessing it might cause some smearing to the sonics. Then again, never tried it.
Tying the cart coils together turns them into sort of an autoformer that bucks the non-signal and forces only matched groove signals to pass. Thought it might end up being quieter too. At least in terms of groove noise.
Make sure the switch is well shielded.
jh
