You don't really need to spend over $300 to get a most excellent closed back headphone... the Modhouse Argon Mk3, which is closedback planar headphone that I like over any other planar I've tried and even my Fostex TH-X00PH... You just need the amp to drive them properly. Bass quality is second only to my STAX L300 Limited... really deep extension, quick speed, excellent note separation with extraordinary detail, really punchy presentation, and with a sound scape that is reminiscent of listening to speakers but not as mid-forward.
My modded TH-X00PH has an absolutely bloated mess of low end compared to the Argon, and it's noticeably better than a stock Purpleheart. The only thing the Purpleheart does better is subbass presence with a massive sound/energy unlike most anything else. Ever since getting the Argon's I'm struggling to think of another closed back I have liked better. For technical abilities such as speed, clarity, punch, dynamics, and resolution I would slot the Argon in between the HD58X and modded STAX L300 Limited, actually being much closer to the capability of the STAX Lambdas. It's pretty much similar to a Focal Clear, but with more punch and low end. Just need to properly power the Argon to have it show its real potential, will sound lacking and flat when under-powered. Will go from flat planar sounding to an extremely good dynamic driver type sound quality when an amp is proper for it.
I really liked and considered eventually getting the Campfire Audio Cascades after having a 10 day audition with them earlier this year, with close to 100 hours of listening time. They are very nice portable closedbacks. But the Argon's have changed that... no inkling whatsoever to pick up the Cascades anymore.