Have you listened to a 909? If not, you might find they're not for you. Personally I adore them, but they can be bright with a bad pairing amp/dac. I didn't get on with them with my old Schiit Jotunheim, but there fantastic from my Chord Mojo 2.
If you do like the sound then try a used pair of Beyerdynamic T1 gen 2. Generally get them in good to excellent condition for around £250-£350. They have less bass impact, but they share the holographic/bubble type sound of the 909.
They're not the 909 (nothing is), but they're a similar headphone, lighter, easier to get aftermarket cables, and are an excellent headphone in their own right. Also play superbly well with tubes. They're a big upgrade from the X2HR. The DT1990 is also a worthy upgrade. My first decent headphone was the X2HR and I still keep a pair now.
You could also keep your eye out for a Grado GH series (a Grado with some bass). You don't see them for sale often, but they also don't command crazy prices (bit more than T1.2). A quick detachable cable mod and some shipibo earpads and you've got a real gem.