I still miss a few things in these HTF600 compared to XB500 but when I put on XB500 I miss a couple of things from the Panasonics. I'd want XB500's bass, soundstage and comfort with HTF600's mids and highs.
The mids are so amazingly forward at all times especially with the mod but the bass is like tooooooooo controlled & clean for my taste sometimes depending on music I listen to, no matter how I boost it, it always just stays controlled and doesn't turn the headphone into a mess, mids always just stays clear and up-front which kinda amazes me but it's the basshead in me that sometimes want to enjoy a bit more "uncontrolled" bass too, it's like too much of a "headphone"-bass vs XB500 which reminds you more of a subwoofer kind of bass compared to most headphones. For most people this is a good thing indeed and I can imagine how much praise it would get from some people, I've never even got close to a headphone that's capable of bringing so much bass but so controllably, it's insane! Maybe the mod has to do with it, it certainly didn't become weaker, possibly the opposite but probably got a bit more controlled.
The funniest comparision is using the ZO amp where the Panasonics still sounds reasonably balanced and there's no hint of boomy bass even at max bass level and the mids gets out in a very clear way even at max contour level (which I think theoretically adds like 9~13dB boost at the max lvl 31 depending on headphone a bit) and despite you can feel the bass and the earcups vibrating, it just doesn't sound like a mess, mids are there unaffected as ever and stays clear and forward and you don't hear reverb in the bass or anything, it's just strong tight bass but then again there's not much difference, if anything at all, after around lvl 5-6 or so compared to lvl 31. Do this with XB500 and it sounds like you put the ear next to a subwoofer port at the max level, it happily goes overboard and would gladly work as an earthquake simulator if you let it. xD
Well the Panasonics doesn't lack in bass quantity for me (ok I did have to boost the deep bass by like ~3dB to get it even sounding down low compared to the midbass), it's just the way it handles the bass response is not optimal from the spoilt bassheads point of view in me.
Other head-fiers will probably love this controlled bass though! I still like them though, especially for pop music with a focus on the vocals and ballads but there's certain music I wish I could have XB500's bass in it. I think pop is the most suitable genre for it due to the punchy tight but strong bass and very forward mids and reasonably neutral highs.