Whatever works. Normally I'd expect to see the tensioned (well, it looks tensioned) guard membrane add a resonance (or subtract one), but if it measures and sounds better with it as it is, and the CSD plots don't show a horrendous hangover somewhere, then heck, why not? Might even be a useful tweak for orthos, which are usually thought of as needing very little additional protection. Well, except for the YH-1000.
There is a resonance. I posted a comparative frequency response a day or two ago - basically the area around 10 kHz gets some extra energy with the moisture guard. Other frequencies aren't affected much. Today I bought what is advertised as TSPO2 film (apparently polyester); it's meant for roasting stuff in the oven, but comes in a roll so I can avoid the folds. I'll try that and see what happens. Neither that film nor the one I used earlier are as thin as the original moisture guard, but whether that matters, I'm not sure.
A picture of the Unipolar 2002 baffle from an auction (thanks, nick):
Unless they've been modded or otherwise obscured in the photo, it seems that Sennheiser got rid of the acoustic filters on the baffle vs. the 2000, and it also seems like the area sealed by the pads is no longer vented (a plastic ring was added around the inner baffle, inside the vented outer rim). These are changes that I've basically done to the 2000 - removing the side grille and plopping the (AKG) pads straight on the baffle, and taking off the acoustic filters to open a straighter path for the sound off the driver. Sounds better and measures better.
Edited by vid - 11/7/12 at 9:36am