Great comments and info on this thread - wanna say thanks to everyone contributing.
I'm interested to hear from more audio engineers and recording technicians about their experience with the 1990s. And/or those who have had experience with and the understanding of the necessity of a relatively flat-response reference grade yardstick. I'm in need of some reference headphones for music production, mixing and mastering. However - and this is key - I don't want them to sound boring. There needs to be at least some excitement otherwise creativity tends to get very flat indeed. This is why I'm looking into the 1990s as they seem to have a good balance between euphoric listening and neutrality / transparency. Combine this with the option of interchangeable ear pads and the range is even more extended.
As far as frequency response goes, two things are most important to me. Mid-range must be present (this is why I love grados) and low-end extension must be substantial (as I use a lot of subs in production). But, the low end must also be very clear and transparent (kicks to be 'heard' rather than 'felt'). I'm hoping that the A pads will lend themselves well to this, by creating more neutrality in the lows, whilst perceptually raising the mids as a result. Semi-open or open are preferential to closed, which is why the 1990s are a real possibility. Comfort and non-fatigue are also important when working long hours.
Unfortunately, many of the other headphone manufacturers have already written themselves out:
Focal Spirit Pros (on-ear, painful to wear, easily breakable)
Senns (the dreaded sennheiser veil is no good for studio work and I already have the 565s for that brand sound if needed)
Grados (just too darn painful to wear)
AKG (had experience before but never truly impressed, although the K712s might be an option)
Beyer 880s (how do the 1990s compare?)
Any comments on transparency, neutrality and reference-grade standard capabilities would be much appreciated.Thanks folks