Matt, regarding the size and shape of the bulb, I've just switched out the 75W bulb from an old Durst 606 enlarger into my Durst M600, replacing the 150W globe which was giving me ridiculously short exposure times, even stopped right down. I intended to run some tests anyway, but you've just given me something else to look out for. The 75W bulb is slightly smaller and narrower than the 150W. Do you think this might be much of an issue?

I have a Kodak Wratten ND .20 filter which I was intending to use but my concern with it is how stupidly hot the enlarger gets so I thought I'd try the other bulb out first.

(Apologies to the OP if I seem to be hijacking the thread but I hope any answers elicited to my question might help others also with regard to replacing bulbs... that's my excuse anyway )