They are not needed these days with nearly all of todays sources, except maybe with weak tube output sources.
Sheesh. It was nice when george was off. Now we have rehash this again and again. Tape is still very much with high end audio as is the LP. These media are not served by any DAC I know of. Plus there are active line stages that have better volume controls than I'm used to seeing in DACs. Line stages first started showing up in the early 1990s when the idea of an all-digital system became a thing. Rather than a relic, it seems as if they are not going away anytime soon.
If you are going to run a passive control, the best place for it is built into the power amplifier. That way the impedances involved can be far better controlled. But if you have monoblocks it gets tricky.
My front end with preamp has 30 feet of interconnect between the amps and the output of the preamp, which also drives a subwoofer amplifier. In this way I keep the front end of the system at a lower level of vibration and I can run a Distributed Bass Array. These are the sort of things you can't do with a passive control. At best, to make those work you have to run short cables- 30 feet is out of the question. If you need dual outputs, you'll instantly be in trouble, and if you want to be finally rid of that pesky standing wave in your room (causing a loss of bass at the listening chair), you'll need a distributed bass array to do it.
