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Based on my fondness for an old pair of Sennheiser HD595s, which I was using with a Marantz PM5005 integrated, I decided to buy an HD598. First thing I noticed was an incredibly boomy mid-bass--almost unlistenable. This had me stumped, as it wasn't apparent at all with the 595. Finally I decided to try the 598 with the headphone stage of my Beresford Bushmaster MkII dac. Result: no boom at all. Weird. I actually much prefer the sound from the Marantz as it has a great soundstage, but have to use it with bass at 9 O'Clock or it's unbearable. So how come there's boom with the 598 but not 595? The only thing I can think of is that there's some sort of impedance mismatch. The 598 is of course 50 ohms, as you would assume is the 595. However, I seem to recall some of the early 595s were 120 ohms, as mine very well could be. However, I'm just clutching at straws here. If anyone has a better explanation I'd be pleased to hear it.