> Is it possible to make an analog filter where you can continuously
> vary the cutoff slope

There are several designs for what are called 'variable slope filters' out there - Google '"variable slope" vcf' and you'll get a number of interesting hits. I've not ever looked into this in much detail, so I'm not sure just how accurately the term 'variable slope' is being used, but I believe the general idea is to 'mix' both shallow- and steep-slope outputs from a VCF - this sounds like a simple enough idea, but the catch is of course that the phase difference between them has to be carefully controlled, and presumably _that_ is where the difficulty lies in designing such a beast!

Tim