Why don't all phono preamps include a rumble filter?

By the way, if you don’t like the idea of an extra pair of ICs in order to incorporate the KAB filter, you could hard solder the device in-line with your preferred phono ICs. That would satisfy even a purit zealot.

Louis, with whom are you arguing? None of us is against the idea. Many older phono stage designs, especially from japan, DID incorporate a rumble filter. So if you really want that feature, find one of those.  American products from around the 1970s maybe up to the 1990s also can have rumble filters built-in. The lack of a rumble filter on modern phono stages is partly a symptom of laziness among manufacturers, and their desire to cut costs while also raising prices. And finally, the outboard KAB filter is really no different from what you would get if it were built into a phono stage. So, you can add that function to any phono stage, via the KAB filter which you apparently already own. But first of all, I think it is important for you to figure out the source of the rumble. Rumble is just another word for low frequency noises. There are many causes. Sometimes the problem can be cured without resorting to a filter. For example, is your cartridge well matched to your tonearm? Perhaps the resonant frequency of the tonearm cartridge combination is too low, causing the system to resonate due to foot falls or subtle record warps.

What I said.

 There are many purist  types who would reject the notion of using a rumble filter for just the reason that you suggest in passing. Which is to say the filter can degrade sound quality. After all, the simplest filter you can construct would be a passive one that would use a capacitor there is no such thing as a totally transparent capacitor. But also you would need some active components, so that you do not lose gain as the signal passes through the filter.  All of that said, I think KAB make a rumble filter that can be used outboard of any preamp. Check their website. What I myself would do is to find out why my system needs a rumble filter in the first place. Once I have figured that out, I would correct the problem, rather than adding a filter.