IMHO, the HD598 is much better than the HD595, so don't sweat it.
The problem I had with the ATH-M50 was not its sound. It was fine there. What I didn't like was the discomfort. It got a bit better when I put the pads for the Shure SRH-840 on it, but still not quite good enough. It's more expensive, but I find the Beyerdynamic DT770 Pro 80 Ohm to be a much better headphone in terms of comfort and sound than either the ATH-M50 or the SRH-840 (though I think the SRH-840 had the best isolation of the three).