I have both the Senn HD598 and the Audio Technica M50x.
I find the bass is a bit more pumped up in the 598s and because it's open has a more airy feel to the sound rather than feeling like I'm locked in a sound chamber.
Personally I prefer the 598s for music and general purpose use and the M50x for my software/digital pianos, and as a bonus no one can hear my boring piano practice.
M50x is better value. For digital piano, I would recommend the M50x.
Of course to each their own, and whatever they prefer is fine. In my case it is the opposite for piano mostly liking the senns more, but you do not have not take my word word it. Measurements, but more importantly my ear tells quite the opposite about some of what you say.
The m50x has better bass extension, it is also more boosted. It is dangerous to go by marketing spec sheets for that type of thing, for anyone that does, don't. IIRC the 598 quotes a lower frequency for bass limit than the ath m50 but in reality this is poo.
50 Hz testhttp://graphs.headphone.com/graphCompare.php?graphType=3&graphID
The links you need to cut and paste in your browser, since it is gets confused by the square brackets by the software on this site.
I'll not go in too much detail for those interested, but two graphs above tell a lot of the story, in particular the 50 Hz test for the bass, though the m50 does have some unusual qualities in the bass, overall very pleasing sound though I like. The senns are actually flatter overall.
The background what all that means is out there at inner fidelity.
My own experiences with quite a few cans in the last years, calibrating my ear as it were taught me to recognise those patterns fairly well but by no means an expert.
A general summary of different can characteristics can be seen here. http://www.innerfidelity.com/conten...wave-response-page-2#3Cy8BiFYIKpAUUsw.97
This page ( to me ) tells a wealth of information more than what a lot of the reviews out there, but, understanding those plots takes a bit of ear training and digesting some theory. Naturally, the ear and actually hearing is the most important and has the final say for me,
I've found that type of data very useful over time and the indications of particular traits a phone will have. That understanding didn't come overnight, nor did it it turn me into an audiophile snob believing in the most expensive can and headphone amp is everything.
My 2 cents, just to point it out, since people come and read these threads and there is a lot of conflicting information out there.
Of course it is not always that simple and black and white to interpret a sound signature, but there is a lot of false info being spread about many a can that are simply not true.
Now, the character of the m50 while a very nice phone does have the tendency to hype certain frequencies important in the focal range and for pianos, not immediately obvious from a frequency plot but also revealed in the square wave tests, this will shift the timbral characteristics a bit. It may turn out that it may happen to be a good match one may like for a particular sampled piano for personal reasons, but it not so natural and/or true in that sense sounding ( to me anyway ).
It certainly does have other qualities where it is a better phone than the 598 when I use them, measurements also tally with why that is so.