Greetings Alan
It is indeed tough to distinguish the difference between the same model displays if you are sitting at the optimal distance based on resolution. However resolution is not the most important property for discerning image quality. Displays vary considerably in how well they perform and the geek box above provides a partial summary of several important parameters. Not everyone will be sensitive to those properties, my wife for example cannot tell SD from HD while my son seems to know when I change a gamma setting.
ISF ranks contrast ratio, colour uniformity, greyscale and colour accuracy and finally resolution as the most important factors affecting image quality. I would also add luminosity to that list, especially for HT applications.
In my opinion, luminosity and contrast ratio are the two most important properties which distinguish a good from not so good display and are very discernable. Images from a high lumen output projector with a very high contrast ratio will be much more visually appealing that a dimmer display with lower contrast, all other properties being the same. Its the main reason why the much acclaimed JVC-RS1 is on my short list for projector upgrade. Though I have concerns that the JVC will not track greyscale as linearly or have as accurate colours as my existing 720p Sim2 HT300e projector.
Moreover not all displays track greyscale in as linear fashion. If there ever was an analogy to audio, its greyscale tracking with speaker frequency response. The more linear with less delta error, the better colour and black detail reproduction. Not all displays excel in this important performance area which does lead to visible differences in image quality in side by side comparisons. The higher the error in greyscale tracking the more image quality degrades. When it comes to colour, I have to admit to being quite sensitive to proper hue and saturation. Not many displays reproduce 701 and 601 colour gamuts accurately. Overblown reds and unrealistic greens can be are very distracting. Underperforming displays and many displays our of the box have oversaturated colours. Green hair highlights in particular bug me.
Motion artifacts, which are an outcome of video processing, are another area which are visible the larger the screen. The list of video properties/artifacts which are discernable is rather lengthy. Resolution is not that important compared to some of those mentioned above.
That is why I suggest that the main benefit of 1080p displays is not that it is higher resolution than say 720p, its that for the most part there have been technological and design improvements in the other more important properties. In our household, I use my sons who have better hearing and eyesight than me to do blind tests ( no pun intended ) and they seem to be adroit at spotting visual artifacts or inaccuracies.
As for DACs, well you know my view: quality DACs make an audible difference. I was at a friends place last week and we were A/B ing some tunes through a Krell Showcase pre and a Proceed AVP2 and the sonic signature was quite apparent between the two and when we switched digital and analog inputs with each processor.
You ask a great philosophical question and it is difficult to generalize. To me specs and listening go hand in hand and while great specs are no assurance that there will be an audible difference, I'm in the camp that better specs cumulatively usually translate into superior performance.