I cannot really speak about the ATS - I have only seen a few ATS/ATM80 models - all HD. Also I don't know a lot about the objective design of the ATS - Kimmo stated that the ATS80 HD uses a triplet with an ED element in some review - not sure what this means for the non HD version - could be regular glass doublet, ED doublet or regular glass triplet.
When I met some fellow birder who had the plain glass variant of my fluorite doublet TSN-3 (which is a very nice example), we did swap eyepieces around and compare the view and the result was that sharpness beyond 40x was visibly degraded in the TSN-1 using a plain glass doublet.
Since you say that the image is pin sharp at 60x, I would rule out a plain glass doublet. Also sample variation is present even in alpha brand spotters - a so-so example of the HD might be not as good as a cherry non-HD...
If you get the chance to buy an HD version in person, take your non-HD example and compare side by side at 60x... If you see an improvement which is great enough to make you spend $$$, make sure you get exactly the device you tested - not a different example.