Pro 900's. Simply the finest overall headphone I ever owned. (The mids and highs might be more transparent on the 650/600's, but what that gives the Bass comparison takes away. It's easier to tweak the response with the FiiO than live without the Bass on the Senns.....) Which has included Sennheiser 650/600/580/560's and a pile vintage Pioneer's and Koss's. (And, a set of earlier beats, not as bad as some people make out, but not the be all and end all either.) I have been thinking about a set of Sennheiser 800's or 700's but the price just seems completely punitive. I use the headphone outputs of vintage gear (Pioneer SA-9500II) to power Headphones at home. There was a time when home audio wasn't as headphone underpowered, at least not as underpowered as they are now. It drives both the Senns and the Pro 900 just fine. (For that matter, so does my Pioneer CT-93, Dragon, RX-505 or any one of three CT-F1250's, an Oppo 105 or Denon DCD-A100) But I do have a FiiO Alpen E17 (and the docking Station) and a Ray Samuels P-51 for portables and laptops.
I've been to the Can fest in Charlotte for the past several years, and got to hear a bunch of the "audiophile" favorites and most of them leave me just a bit underwhelmed. (Denons, Grados, and those squonky looking Sony's, I never had luck with Sony's in terms of long term reliability anyway...) I actually prefer the "speaker" like perspective of the 900's. Our ears are MOST sensitive around the frequency of our voices (1khz, give or take) and that means that THAT frequency is the one we can tolerate high SPL's the LEAST. So contrary to the usual narrative, I'm okay with a (slight) recess in upper base/midrange. I keep looking for an upgrade, but I never seem to find one that hits the "quality" and "price point" in a manner that makes me pull the trigger. To be fair, I have NOT heard a set of 800's or 700's yet. I'm thinking seriously about a second set of 900's. If it's not broke, don't fix it. I picked up a pair of V-MODA M-100's recently and they're quite nice, but no serious challenge to the Pro 900's (IMO....) Especially if you factor in price. Just my two cents. YMMV.