I agree with AlphaGT and Rothchild, changing EQ is not adding distortion it is changing the amplitude to a sign wave at a given frequency and that is part of the RIAA process anyway.
Distortion in Vinyl play back mainly comes from miss tracking caused by poor arm / Cart geometry or set up so its very important to get that set up correctly first.
It is also possible that harmonic distortion is unintentionally added by a poor quality phono stage adding a bloom to the sound ? remember that MM ads approx 45dB and MC 60dB of gain to a signal so any distortion in the signal path is amplified up to a thousand times
When they talk about about vinyl warmth I just think that is a product of the frequency response produced by the RIAA correction and the Cartridges own Voice (and cartridge loading) plus the distortion in the phono circuit which does not produce the ruler flat frequency response that you get from digital.
Reel 2 Reel also produces a hump in the bass and at low speeds a rolled off HF but it still sounds great never the less
I think that we also have to accept that certain types of distortion do sound good to the human ear and a perfectly pure and flat frequency response can sound a bit flat and sterile
Also remember the every guitar sound we hear on a song has already had all sorts of distortion sounds intentionally added to make it more attractive to the music being played
Alan
Turntable - Garrard 401/Jelco 750L/Ortofon Kontrapunkt B, Pioneer PLC 590, Micro Sieki MA505 , Denon DL103R - DIY Paradise Phono stage - Reel 2 Reel Studer A810, Otari MX55,Tascam BR20, Revox A77, B77, PR99, TEAC X1000 & 3440, Digital HTPC / Young Dac - Preamp - DIY B4, 821, Power Amp's DIY Avondale NCC300 Mono Block, Speakers Wilmslow Kit Volt BM220.8 / Scanspeak D2905/9500