No experience with the kowa 660 and the nikon EDIII, but I can share some experience with the eyepieces on the 60mm Monarch. The standard 3x zoom MEP 20-60 (16-48x, 42°-60°AFOV on the 60mm) will scratch your spectacle lens if you do not raise the rubber eyecup a little because the eyecup is level with the metal ring around the ocular lens at the down position. I had to tie a 1mm string under the eyecup to overcome this issue. The listed eye relief of 16.1-15.3mm is misleading. Eye relief at 16x 42°AFOV is good but decreases drastically when you turn up the magnification, especially at mid magnification range 24-40x (not useable for me) and then increases a little at 40x towards 48x (useable). Hence for a spectacle wearer, the 3x zoom is practically usable only near the 16x and 48x magnification ranges so don't bother with this eyepiece.
The fixed 30x magnification wide angle eyepiece MEP 38W is nice with ample eye relief for spectacle wearers and no issue with lens scratching due to the large ocular lens and very narrow metal ring around it. You can view without your spectacles touching the rubber eyecup. Very comfortable and immersive due to the large AFOV.
Unable to curb my curiosity after corresponding with Ruff-Leg, I got hold of the 2x wide angle zoom MEP 30-60W (24-48x, 60°-72°AFOV on the 60mm). I didn't like it initially. It is big and cylindrical, without the taper at the end like the MEP 20-60 and the 38W, and heavy. Perhaps I am too used to the eye relief and simplicity of the 38W eyepiece since to use this 2x zoom, you more or less have to rest your spectacles on the eyecup. Thankfully, the eyecup on the 30-60W is like those on binoculars with elevated rubber surface at the edge of the eyecup. Again the listed 15.2-14.2mm eye relief is misleading, but in a good way. With my spectacles I can see the full 60° AFOV at 24x and the view remains usable through the higher magnification range. Beyond 30x, a little of the edges gets darkened/cut off and still acceptable. Optical resolution appears to be a little better than the 3x zoom, but I don't use the 3x much and not an expert. Star test with it showed effectively circular discs under and over focus over the whole magnification range, though when the star moved too far off the center, the outer disc gets cut-off which I assume is due to a field stop in the zoom eyepiece. Here is Henry Link's impression of the 30-60 zoom eyepiece:
The 30-60 zoom does have a nice wide apparent field throughout the zoom range. I didn't have time to measure it, but comparing it to eyepieces I have measured I would estimate the true AFOV at 30x is about 58º and around 73º at 60x, as expected somewhat wider than the Nikon ISO specs. Distortion appears to be moderate pincushion, so no Globe Effect to deal with. With one exception off-axis performance is very good. There is a little field curvature and astigmatism at 30x, which gradually reduces to a very low level at 60x, The weakest off-axis characteristic is lateral chromatic aberration, particularly at high magnifications. At 60x there is an area near the center about 20º wide which is mostly free of lateral color, but for high contrast targets outside of that area the color fringes come on quickly and are pretty vivid at the field edge.
Having used the 30-60W eyepiece for nearly a month, it has grown on me and I still wished that the eye relief is like that of the 38W eyepiece. Note that all three eyepieces experiences reflections at the ocular lenses when mounted on the angled scope variant if you have a bright sky behind you. Might produce a white arc in photos/videos if using your handphone to capture without a proper adapter.
In the unlikely event that you decide to get the
heavy 60/82mm monarch scope, skip the 20-60 eyepiece and get the 30-60W instead. The 30-60W more or less renders the fixed 38W eyepiece redundant, but the longer ER, 75° AFOV, smaller size and weight of the 38W is still highly appreciated by me.
