I have both of Senn HD-6XX and the Fostex TR-X00 Mahogany's (and a handful of other mid-fi priced sets: Sennheiser HD1 on-ears, HiFiMan HE-4XX, B&W P3 Series II, HiFiMan Edition S, Audeze LCD-2c, etc). Of the sets I own, the two in question are two of my three favorites. I like both a lot, and use both a lot. But they're very different sets for very different use-cases.
When I'm at home, in my sunroom, tethered to my best amp, with a beverage as a sidecar, I think the HD-6XX's outperform the TR-X00's on sound quality. They image better (open-back design). They have a wider sound-stage (open-back design). And the tonality and fidelity with mid-range and female vocals is the best of all the gear I own (speakers and headphones alike).
You'll commonly see on the internet that "the 650's (and 6XX's) scale nicely with equipment" and that has been my experience as well. When plugged directly into a phone or laptop and A-B compared, the TR-X00's significantly outperform the 6XX's (and it's not close). Off of a cheap/portable DAC/AMP I own (Fiio Q1), the 6XX's close the gap a little, but still come up well short of the TR-X00's. Using my Schiit Jotunheim via the stock single-ended cable that comes with the HD-6XX's, and the 6XX's start to outperform the TR-X00's for pretty much everything other than modern hard rock and hip-hop. With a 3rd party (periapt) balanced XLR cable and using the Balanced output on my Schiit Jot, I think I pretty much prefer the HD-6XX's in pretty much every way (sonically) over the TR-X00's.
But the TR-X00's look great. The Sennheiser HD-6XX's are not appealing, aesthetically, to me.
The HD-6XX's are not uncomfortable, but the TR-X00's are the most comfortable headset I own.
The TR-X00's are a closed back design that are optimized to run off of "standard power sources" (my work computer, for example).
All of this means I can plug my TR-X00's into my work PC and stream CD-Quality music off Qobuz, without my co-workers being bothered. And I can wear them for hours after hours at a time, comfortably, and feel confident that I like the way they look when I'm around my colleagues. I can't do any of those things with my Sennheiser HD-6XX's.
So I like both. A lot. I use both. A lot. I use the Fostex's more, because I probably listen to music in the office for 20-30 hours a week, and don't own any other closed-back headphones that come close to these, performance-wise. I use the HD-6XX's less because I probably only listen to music 3-6 hours a week at home, and when I'm at home I have a lot more options to choose from (two 2.1 speaker rigs and pair of Audeze LCD-2C's are the Sennheiser HD-6XX's main competition). As a result, the TR-X00's get a lot more use for me, even though I think the HD-6XX's sound amazing.
Hope this helps.