If you can afford two pair, you really ought to look into the HD600. They are technically superior to the HD6xx in definition, neutrality, and accuracy - my best descriptor is "surgical" - a quality that may fairly be called "cold" or "sterile". Their worst quality is the lack of bass, but their treble is so clear. Fanboys have told me that the bass level is perfect, but since (everyone and) I have listened to boosted bass all my life, I wouldn't know it when I heard it. Maybe they're right. (shrug)
I've got all three Sennheisers. The 58x simply don't stack up to the par of 600 and 6xx. They do have the best bass, but just barely at that threshold where I would feel comfortable assuming there is general consensus here without asking. Basically, what that means is that the bass volume is slightly increased, probably the bass extension too, and its quality is not reduced compared to the others. Everything else about them is sacrificed by comparison.
I have had and loved the HD600 for a decade, and I had never heard anything better in that time, but when I got the 6xxs, I put the 600s on the edge of my desk, behind the monitor. I listen to the 6xx 90% of the time, preferring the warm "laid back" quality that never causes me ear fatigue and the comparably prominent bass. I take out the HD600 once in a while reliably confirm that in terms of "listenability," the 600s are outclassed across the board. But I will never sell them for the same reason I told to you.
For the last month, I have been using the Darkvoice. One thing that may not be obvious about the Darkvoice online is that it is beautiful. It is the kind of thing you might purposely place in line of sight - a conversation piece with charm similar to a record player's. I'm told the stock tubes that come along with it are low quality. But I'd take my samsung S8 plugged direct into the DV rather than going through the $150 SDAC/O2. Now I'm about to pull an M. Night Shyamalan - hope you're sitting down.
From ebay, thought I'd take a little gamble that probably wouldn't pan out and got a tube shipped to me for $14 to swap with the little tube in the front. $14 is typical. It was the RCA 6SN7GTB. I had no idea what to expect, but this tube cranks up the bass.
The RCA seems to make the 6xx muddy or something, it's actually kind of worse for them. The HD600s are absolutely wonderful. I can't put em down. They sound like the 58x bass got slapped onto em along with a touch of the 6xx's laid back personality. Okay, now I've written exactly 500 words, so bye.