I've made a cascaded passive 2-pole pomona box for fast ALS use, using LISO to check that it'll give the right shape when hooked up to the CM board's input stage. 

First stage is a 133Ohm + 10uF cap for ~120Hz LP, second is 1.15kOhm + 47nF cap for ~3.8kHz LP. The DC gain is ~0.75, which is much better than what I was doing before. The second stage would normally make a 2.9kHz LPF on its own, but the loading of the input stage moves the corner up. 

It seems the 133 Ohm resistor is a reasonable load on the output AD829 of the ALS demod board (short-circuit output current of 32mA and a series output resistor of 499Ohm). To be able to use the digitized ALSX I and the lowpassed analog version simultaneously, I had to buffer the signal with a SR560 before the pomona box, otherwise the signals looked distorted. This isn't a good long-term solution. Maybe I can used the further-buffered differential output to drive the LPF+CM board. 

The LISO files used to model the filter and CM board input stage, and fit the pole frequencies are attached. 

I made some attempts to get the AO path going today, but I suspect this daytime noise is just too much; the PC drive seems too irritable