Good question, Rich.
About 8 months ago the passive crossover in the right speaker of my extremely expensive, pristine and very accurate Dynaudio M5P speakers had a little problem. Turned out to be elsewhere but during the process of elimination I had the crossover apart and so my mastering loudspeakers were down. During that week, while the debugging process proceeded during off-hours, with some trepidation, I switched to the following for mastering monitoring:
1) My pair of Audeze CRBN Electrostatic headphones (around $5000 if I recall)
2) Driven by a $3000 Mjolnir Electrostatic headphone amplifier
3) Processed with my Weiss DSP 502’s crossfeed algorithm calibrated by me to produce a stereo image image equivalent to the +- 30 degree separation of stereo loudspeakers
4) Equalized in Acourate Convolver with my custom EQ that I had designed to subjectively replicate the response of my reference calibrated loudspeakers. This EQ is EXTREMELY accurate tonally from the lowest to the highest frequencies and the electrostatic headphones present extremely linear response with no obstacles between the lightest diaphragm in the industry and my ears.
I was extremely successful in mastering with the above. Mastered an album and a few singles in the rock and singer-songwriter genres and nailed them! The clients were very happy with the results. To be safe, I checked my masters upstairs in the mix room Studio B on the mind-blowing Kii 3/BXT loudspeakers, which also are quite precisely equalized in a well-treated room.
(Why didn’t I try to master in Studio B, then? Because I have all the tools and my racks for mastering in studio A and Sequoia is geared for mastering, so I really would have felt handicapped trying to mastering in Studio B.)
Anyway, I am happy to report that you can master in stereo with absolutely superior headphones whose frequency response (equalized) you absolutely trust as a reference, and with a crossfeed algorithm whose stereo image produces a +-30 degree location for the left and right channels, and years of experience under belt. The cost of such a system is not trivial and rivals the cost of a set of mastering loudspeakers. I would not recommend mastering with anything less. In my opinion, planar magnetic headphones are a bit too wonky in some regions due to the interference of the Fazors to the stereo image and frequency response, and I haven’t found, equalized, that I would trust any of the Audeze that I have to use for mastering, unless that’s the only monitoring that you have available: And I have Audeze: The Marroquin MM500, The LCD-4, The LCD-4Z, The LCD-5. So I know the weaknesses and strengths of each one and each of the EQs I have developed for these have truly improved them but not in my professional opinion good enough to do A grade mastering. I could see doing “B grade mastering” using the LCD-5 or the MM-500 if push comes to shove and you use a great DAC, great EQ, and a crossfeed algorithm.
Hope this helps.