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I didn't compare the Pentax 14mm, as my comparison was only between the 14mm Meade and the 13mm T6 Nagler.
However, the Meade has 8 elements in 5 groups: 10 air-to-glass surfaces, and some of them appear to be completely uncoated. And some tests show transmission below 90%.
The Pentax 14mm has 7 elements in 6 groups, for 12 air-to-glass surfaces, but all are super multi coated, and the transmission of the eyepiece peaks at 96% at 550nm (green) which is nearly theoretically perfect for that many surfaces. All tests and observers have commented on this, so the difference between the XW and other widefields is likely to be real (but see my later comments).
The 13mm Nagler T6 has 7 elements in 4 groups, for 8 air-to-glass surfaces, all of which are multi-coated. I would expect transmission to be closer to the Pentax figure than the Meade's.
Though I believe people see differences between these eyepieces in regard to what they see, it may not be transmission that defines the difference. If it is, I have no explanation for that because controlled lab experiments show the human eye to be nearly blind to a difference in brightness of 10% (about 0.1 magnitude, which the AAVSO also holds as nearly the minimum identifiable difference under exceptional circumstances), so I suspect the differences are related to other factors, like surface polish, scattered light, exit pupil issues, internal baffling in the eyepiece, the frequency of greatest sensitivity of the eye vs. the eyepiece, etc.
But, if you add a small difference in transmission to those other factors, it probably adds up to what people see and have commented about.
The 14mm Pentax, for what it's worth, and the 40mm, are the only 2 XWs that I've never looked through.
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