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I would only buy the Denon if I wanted the absolute best video from one box available.
The Onkyo SP1000 is a better audio player, same build quality and just behind a bit on video.....maybe 5%.
The main thing the 5910 does is make not so great looking dvd transfers look better as compared to these other players.....maybe 10%.
On very good to great dvd transfers were talking maybe a 5% subjective difference between the 5910,SP1000,79avi.
A Pioneer 79avi using 480i via hdmi to a good scaler such as an Iscan ect will probably be better yet for just video on great transfers for less than the 5910 list cost.
It all ends up depending on your budget....as well as whether you want the best video only...using a seperate player for audio....or if you want the best compromise of the two in one player for under 3.5k.....or whatever your budget is.
I use a pj with over 100 inch screens so the differences in these players become more evident imo...and on screens under lets say 70 inches these differences may lose some value.
I compared a highly rated Panasonic s-97 to my current Onkyo SP1000 and could see a subjective 20-25% difference in background noise and detail...as well as color saturation.
Comparing for yourself in home, under calibrated conditions is always the best way to go imo...if you want the best for your set up.
Tje 5910 is the current best one box solution for video...but only you can answer whether its worth its cost over the other players.