Trust Schiit Loki

Good choice I think. I too use one in my dedicated headphone system and the sound it produces (the headphone system) is outstanding with just a minor twist of a knob. I also use one with an inexpensive DAC and good quality deck and it too is improved. I have not used one between and amp/preamp so I can't endorse that. I have found it most useful in tailoring the upper mid range which is so frequently too forward.

I basically have one setting that I use for bright sounding sources.
I boost  the bass a bit and lower the two upper frequency controls a bit.
This works for most problems.
  I don’t get involved in customizing the sound for each signal.

I also have one for my TV - movies and low level evening listening it just gives that little bit of extra bass in the evening. only complaint I have is the excessively bright LED on the front.

I also have one. Purchased it when I upgraded my old system. For 20 years I used a Yamaha eguilizer, you know the one with 20 slide bars. Never knew if I had it tuned in properly, drove me nuts. When upgrading and reading comments here I learned with good equipment you don't need an equalizer. So true, but 1% of the time my Loki really helps. I all ways forget that that I have used it then wonder why my music sucks, always forget setting it back to neutral. Now I place a big piece of cardboard against my rack to remind me.

The "best" signal is such a temporal thing. It is guesswork even by the original engineers.

They are trying to guess what equipment you will hear it on and what will sell the most.

The idea of being a purist, and instead of using tone controls swapping speakers and cables to achieve the best reproduction is absurd.

My problem with tone controls is only their implementation. I've had 2 high quality preamps, one of which had tone controls that were clearly audible, and my current one they are not. I mean by this the sound turned veiled and lost dynamics. I wish all tone controls were great, they aren't, but worth pursuing.