OK, I'm returning to this forum after an interval of several months and was surprised to see this thread is still active.
In real use in New York, which is where I am based, differences in real world *sound quality* between tuners are negligible because of the abysmal quality of almost all FM broadcasts here. IOW, the tuner - any decent tuner - is better now in this environment than the signal being transmitted. I expect this is pretty much the case all across the US. Thus any comments made about the sound quality of high end tuners on actual US broadcasts should be taken with a grain of salt. I would say that the differences between a TU 919 and TU X1 in real world use are not worth bothering with, assuming a good roof top aerial. The Kenwood 917 has an FM modulation meter and it typically shows a dynamic range of 6dB - yes, SIX dB - on most commercial broadcasts. One so called 'quality' station shows about 12 dB dynamic range.
Can't speak for the quality of European FM broadcasts, but, in general, I would doubt very much any comments on sound quality without knowing the actual quality of FM transmissions.
Of course the foregoing refers only to so called 'sound quality' differences on commercial FM broadcasts. There are significant differences in the RF ability of tuners: selectivity, sensitivity, quieting, and so on. Then there is the matter of ergonomics, the user interface, long- term reliability, parts availability etc. Given current FM transmission standards I would suggest that these are MORE important in selecting a top tuner for long term ownership than so called 'sound quality'.