You can lower the resistance. The tradeoffs are the loading on the differential signal source, the current passed at 230 VAC at one input, and the effects of the RC pole.
It occurred to me that the circuit with ungrounded diodes limits the differential voltage when 230 V is applied but does not limit the common mode voltage (> 80 V peak). If you replace the diodes D1 and D2 in the top circuit I posted on 3-28-2011 with back to back diodes to ground, then you get limiting for both polarities of signals without the forward biased conduction at low voltages. It takes 4 diodes rather than two, but that is a small price to pay for the survival of the instrument.
Lynn
