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I agree with a lot of the advice about looking at the optics, defocussing the image and rotating the eyepiece before blaming the eyepieces.
Another thing to check is whether too much force being applied to the secondary. Depends on the design. This could be from side clamps or it could be from too much pressure from back packing behind the spider if it's a tube type spider.
Some might disagree with me but I think that a high power eyepiece, even a medium power eyepiece is a more critical examination of optical defects than photographs are. It seems highly unlikely that a set of six eyepieces of different focal lengths will all be identically made with the same triangular distortion.
Finally, if you do find you have to buy new eyepieces, don't buy so many. You really don't need that many. A long focal length instrument builds up magnification quickly and possibly needs more eyepieces so as not to jump big steps in magnification but with an 8" newt you can make do with a lot less. You've already noted that you mainly use two focal lengths. Spend a little more on those two focal lengths and forego the rest. Another strategy is to buy a good quality barlow or Televue powermate. Then buy eyepieces that are not focal length multiples same as the barlow. For example, a 2X televue powermate and a 40mm and 13mm eyepieces give you 40, 20, 13, and 6.5mm equivalent focal lengths. Note that switching in a barlow or powermate is less convenient than having parfocalized eyepieces.
Good luck
Joe
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