Hey everyone, I thought I'd put my listening notes here after getting the chance to listen to the Jubilee's and the 6XX's side by side. Thanks to a generous friend who let me borrow his HD 58X Jubilee's. Please take this with a grain of salt as I am not an audiophile. Just someone who enjoys good music through good cans :) My notes to him are listed below and hopefully this helps others in making a decision. I was a surprised by the results, and completely understand that the outcome could be a bit off by using a less than optimal setup with an E17k as my DAC. Anyways, here you go and hope it helps.
My impressions after running on the exact same setup and listening to the same songs. (Yes, I know, I should have included HD/FLAC files). Fiio e17k passthrough/DAC>Magni 3>No EQ adjustments>Highest quality Spotify streaming setting.
Jubilee - Brighter sound with a bit more V shape (mids recessed just a touch) and more precise/cleaner sound. The bass is far cleaner and separates from mid-bass very well. So, you really hear the low end as a separate band when it hits.
6XX - A bit bigger sound stage, a bit more detail, and a more neutral sound with the mids/accuracy being the highlight for sure. Vocals really shine in these. There may be a bit more bass, but barely noticeable and it bleeds into the mid-bass more so there is less separation in that low/mid band, more muddy than the Jubilee's (still really good though). I did somewhat hear the "veiled" sound in these initially where the Jubilee's are far from veiled. That seems to have faded after about 20 hours of listening. Call it burn-in, or psychoacoustics but, it's what I'm experiencing, that the veiled sound seems to have brightened up a bit.
Overall, they are both amazing and after a lot of time on the 6XX's now, I've really taken to them. Simply because you hear absolutely everything and you can listen to them for super long sessions/they aren't harsh. The Jubilees on the other hand, they help you actually feel the music you are listening to and the electronic/hip-hop I listened to sounded better overall (not by a lot though). There's something magical about how clean they sound, especially in the low end (this shocked me a bit)
Both are awesome cans (the 6XX's taking acclimation of psychoacoustics/burn-in for me to really enjoy, and the Jubilee being amazing to my ear out of the box).
Another note, the Magni 3 has plenty of juice to power the 6XX's just to confirm what many others have said.
I'd pick the 6XX's at this point but, I think most people would be happy with either and not needing an amp for the Jubilee's is a plus.