Believe it or not, i actually agree. The KEF M500´s are better cans in terms of high fidelity. More linear, more realistic sound, because of the less bass boom you mentioned. More "honest". what you have on your recordigs, you will hear. This is positive. Or not? I remember, when Mark Levinson created his passive equaliser, the Cello, in the 80´s or 90´s, sorry it was ages ago, he did it on a purpose. He said he knows some hifi freaks with mega expensive systems, and they listen to only 5 cd´s... I felt the same with the KEF´s. I´ve got about 120 albums on my Fiio X3 II´s, and i noticed, when i use the KEF´s, i listen to about 10 albums, the ten best recordings (not necesseraly the 10 best in terms of music!), cuz in case of the others the KEF brings out all the flaws, For example, the album, Blind Melon, sounds really flat. As it is originally, basically. Realistic, but not enjoyable. But when i change to the Sony´s, i can listen to all of my albums, even i can enjoy the worst ones. So, basically i sacrifice the real high fidelity, just to listen to more music. But, as the KEF´s are really good, I´ll keep them anyway, won´t sell them for sure. But i still think that the Sony´s are more versatile.
I like your thinking and agree, its like the HD-650 and HD-800, the HD-650 will sound good with mediocre recording and sound great with great recording while the HD-800 is not very forgiving of sub par recording. The MDR-1A is pretty forgiving of sub par recording but still try stay away from bass heavy stuff when using the MDR-1A.