I love passive filters, and passive circuits in general. Both because less cables floating around and therefore often being able to be used across different platforms/functions, but also I quite like how they can offer kind of unpredictable results since they will behave differently in different contexts. Even just RC filters I find come in really handy. There is a German synth builder, Trouby, who also is pretty obsessed with these and has lots of filter clone modules mostly copies from old DDR organs. They are pre-built but I think he sells them as kits sometimes too, might be worth contacting him - http://trouby.bplaced.net/modular/rcfs.htm. I’ve got the guitar and the ET-6, but older passive versions. Of course you can start just by experimenting with some resistors and caps, maybe some pots if you want them to be “playable” but don’t want to muddle around with expensive hand-rolled switches.
The main thing I tried to figure out and only did a bit of experimentation with was trying to figure out if using things like NoS styro caps actually made a difference in RC filters. I got a mix of old styros, other old film caps, tropical fish, and tried doing some tests and close listening but couldn’t notice any really huge differences, but this was in the context of just using aux amplification and not in a very careful or scientific way, just me goofing around. Given how passives behave I wouldn’t be surprised if things like the tone or non-linearity become much more noticeable and important when you introduce higher voltage and amplification, or where in the amplification/signal chain certain things are placed, like guitarists obsessions with tone caps. And as @GrantB points out, the inductor in these more desirable/classic filters probably plays a much bigger role than I think the caps do. That Altec clone looks really nice… I do wish now I didn’t know it existed though haha. Also what mixer you use if you aren’t using any other amplification will have a big effect. I recently got a new smaller mixer and tried out one of my passives on the aux rather than directly on the input… really bad results compared to my old mixer. I’m also still super bad at the very basic math behind all of this, but wouldn’t the phasing depends on the transformer and what taps are used on it?
Related to all of this too is using transformers for things like saturation and color. Well known good sounding transformers can cost big $$ because of people wanting them for things like studio gear as well as audiophiles for phono preamps. But you can use the aux out or an outboard amp/preamp and send the signal through transformers to get their color and saturation effects in a more crude DIY way too. If you have an electronics supplier near you might be worth doing some digging. I don’t have any that are particularly rare or desirable but I managed to find some for about a euro a piece at a shop I used to live near and collected quite a few different pairs, and even bought up an entire box of custom wound ones which I believe were made for a radio project. I keep meaning to make a project for them and do a small run, maybe someday…
