My first thoughts as I settled down to watch the 55PUT6400 do its thing weren't especially positive.
Out of the box, with all but the most pristine of native 4K UHD content its pictures just don't look very natural or involving, with too much evidence of various sorts of noise, as well as some slightly unnatural colours.
Fortunately I've seen enough Philips TVs over the years to suspect that many of these initial problems might be fixable. And so it proved, as a bit of time spent tinkering with various corners of the TV's picture processing settings quickly ended up producing much more satisfactory results.
For the record, the areas I'd recommend you spend the most time playing with are noise reduction (I'd suggest switching this off for all UHD and most HD content); the Natural Motion system (which should never be set higher than its lowest power setting, or else turned off entirely); the Contrast mode (which should be set to Standard rather than Best For Picture or Best For Power); the Ultra Resolution system (which I would recommend turning off for most content); and the Dynamic Contrast feature (which I would suggest you only use on its Minimum level).
With the changes above made, the 55PUT6400's pictures suddenly become not only more palatable but even, for much of the time, actually rather good.
They're particularly effective with native 4K content. With such an ultra-pure source the screen is able to give full expression to its colour, sharpness and, to some extent, contrast strengths.
Where colour is concerned the screen is capable of good levels of blend subtlety for such an affordable TV, meaning there's minimal evidence of the sort of striping and blocking artefacts you get with less able colour performers.
Nor is the impact of the 4K UHD resolution lost in any 'gaps' in the 55PUT6400's colour resolution.
Sharpness really is intense. This has long being one of the most potent parts of Philips' TV picture processing systems, and it results here not only in upscaled HD images that look sharper than most but also 4K images which even at times look slightly more than 4K!
Actually, with the Ultra Resolution feature active the sharpness can go too far, to the point where images start to look noisy. Turning this feature off, though, reduces (though not totally removes) the noise without excessively reducing the sharpness.
The 55PUT6400's sharpness doesn't reduce heavily when there's a lot of motion in the frame either - especially if you call in the set's Natural Motion processing on its lowest level (anything higher results in excessive artefacting).
Contrast is strong on the 55PUT6400 too for such an affordable set.
Black levels are engagingly (though it has to be said, not emphatically) deep, and the screen manages to retain good levels of uniformity in dark areas even when they sit right alongside bright ones.
There's a reasonable amount of punch to the bright elements in dark scenes too, showing that the screen isn't having to take too much overall luminance out of its images to deliver a convincing black colour.
While the 55PUT6400's pictures are much improved by careful tinkering with the many set-up options, though, there are a few residual issues.
First, while you need to use the TV's dynamic contrast feature to stop dark scenes starting to look grey, this feature can also cause moments of distracting light instability, even if you stick with its lowest power setting.
Colours sometimes look a touch over-saturated in relatively dark parts of the picture too, and grainy source footage occasionally looks a bit noisy in a way I wouldn't have expected to find had the 55PUT6400 used a more powerful Philips processing engine than Pixel Plus HD.
The 55PUT6400's processing engine can cause some ringing around sharply contrasted object edges too, and I occasionally felt aware of slight vertical light inconsistencies during camera pans across bright content as well as traces of moire noise over some areas of very fine detail.
Standard def and gaming
I should say for the record, too, that the 55PUT6400 struggles a bit with standard definition content, tending in particular to lose colour accuracy as it tries to convert standard def sources to its native UHD resolution.
But then actually no 4K TV I've seen is entirely at home with standard definition content.
One last element of the 55PUT6400's picture performance to consider is its input lag - the time it takes for the TV to render images having received them at its inputs. My tests delivered a figure of around 50ms on average when using the TV's Game picture preset, which is a little higher than serious gamers would want to see.