The only thing that hasn't been mentioned here (unless I missed it via a quick re-read) is the massive phase shifts that occur at the boost/cut frequency.
No. The EQP1 is a minimum-phase piece of gear (as most audio equipment); the phase-response is closely related to the frequency response. The LF control circuit are both 1st-order (6dB/octave asymptotes). The attached graph shows the phase-response at max boost and max cut; phase shift nevers exceeds 50°; no one can call it massive by any standards.
Being a largely digital producer, I often insert an 18dB or 12dB VST EQ HPF at sub-sonic frequencies (around 15-25Hz), in order to try and shape the phase response of a bassline so that it sits better in a mix.
Most of my tracks are HPF'ed, but not because of whatever effect it has on phase; I do because I know that anything below 30Hz has 99.9% probability of being garbage, and I don't want this garbage to create havoc with the detectors in compressors or with the loops in reverbs.
In almost all cases, the result sounds "better" (subjectively) than the original.
No doubt about it.
Sliding the frequency up and down a little bit of the HPF produces varied phase shifts that can "lock things in" so to speak.
Just worth mentioning - since I think that this is the REAL effect that the engineers are hearing, not the typical amplitude action of the EQ.
You are entitled to your opinions, but they certainly don't resonate with the most advanced results of scientific research on the subject. The hearing process has no way of perceiving phase of a single signal in the absence of a reference signal i.e. an original non-shifted version of the signal), and even then, the phase difference can be perceived only by the resulting interaction on frequency-response, by a drastic change of peak factor that would trigger non-linearities in the transmission path or by the time-difference in the case of binaural listening.
Also, intriguingly, the shapes formed by basslines from digital synths after the HPF are much more like those coming from an analog instrument, than a standard VST. I feel that this is due to the phase response shearing that is more typical of analog gear.
This is due to the big difference between ADSR in a digital instrument and the natural dynamics of signal when it is produced by a somewhat random combination of fingers, metal, wood, air, analog oscillators...