I'm partial to the Sennheiser sound as well. I have HD598, HD558 (no foam mod), HD599, HD579, HD569, HD598Cs (same as 569), HD600, HD650, HD6XX, HD660S, HD700, and am waiting for the HD58X. While I like the HD598s, they have their limitations. The 598's have built up a cult following, but it's filled with a lot of people who aren't willing to pay for the more the more expensive HD600/650's so they use convenient excuses to convince themselves that the HD600/650's aren't superior. I went searching for something better when I realized the HD598's are a rather poor headphone with a lot of genres of music (even when compared with their replacements the HD599). Stick with jazz, classical, folk, singer/songwriter, some country and other similar stuff and you're fine, but go for some rock, metal, rap/hip-hop, Dance/Electronic/DJ, etc and I find them very limiting. The HD598 sold me on higher end open headphones (until then my only open cans were Porta-Pros), but then I purchased the HD600's. It was then that my love for the HD598 turned to like. It's true that the soundstage with the HD6xx line is smaller unless you have the right DAC/Amp. I recommend BH Crack, Woo WA3, or Little Dot MK II(great if you're on a budget). But the HD600 is still next level gear by every other measure when compared to the HD598. The HD6XX is warmer than the HD600, but the sound is also more refined. I haven't seen anything about how final tuning of the HD58X will sound, but I'd imagine it'll be brighter and more forward sounding than the HD6XX. The HD58X would also be a better fit with your FiiO; you may experience the Sennheiser veil if you did the HD6xx. If you went with the HD6XX and didn't want tubes I'd suggest picking up a Schiit Magni 3. In my opinion it's the first Magni that really pairs well with the 300ohm Sennheisers. It works great with the 150 ohm HD660S' as well. Hope this helps...let me know if you have any further questions.