This is a great thread with lots of meaty opinions ... a fine read.
I agree with the following points, made by others:
-- For critical headphone listening, where the purpose is analytic, as opposed to kick-back, do not use an equalizer. Find the right headphones and amp combo. When just enjoying the music, add any effect you like!
-- Aphex 204 is a fun box, really can add sparkle, just keep the effect level low.
-- A high-end graphic equalizer is a joy, does not add audible noise, and is perfect for eliminating tape hiss from a fave vintage recording, especially with phones that peak hi freq (GS1000, for instance). I have dbx iEQ-31.
-- Room correction DSP, especially with auto algorithms, is a must for odd-shaped rooms and speaker placements. I have Lyngdorf (and a very odd shaped room, plus problematic speaker positioning).
None of these subsitutes for the other: Aphex punch, EQ, room-correction -- all different.
I have balanced cables through-out, and can quickly build different audio chains on-the-fly. (1) CDP/DAC to HP amp with nothing else. (2) Add Aphex to the chain. (3) Add dbx to the chain, after Aphex, or by itself. (4) Replace HP amp with Lyngdorf room correction DSP/amp, by itself, or with Aphex, or with dbx, or with Aphex first, dbx second.
In actual fact I really only use three modes: (a) nothing but the HP amp, (b) HP amp with dbx in front of it, (c) Speakers with everything: aphex, then dbx, then room correction. I never fiddle with the room correction, it is set, and I never fiddle with the Aphex when it is on -- I have "just a little effect" settings for both top and bottom that work well for me. I use the dbx sliders for the treble cut to eliminate hiss, or maybe a little more tweaking (and maybe an Aphex tweak now and again, I lied a little in the previous sentence).
I trained the room correction without either the Aphex or dbx -- that's important, so is putting the dbx after the Aphex.
Enjoy! It's a hobby, do what you like.