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Heya,
The HFI2400 is one of the better Ultrasones to me, the treble is less piercing due to being open, the mids are decent, and bass is very warm, it's bassy, but not over the top like the PRO900. The SRH940 has more detail, better mids, brighter in general, but way less bass response in all aspects. Also the SRH940 is built less durable than the HFI2400. The PRO900 over the HFI2400 is not an upgrade or side-grade, just another flavor. I consider the HFI2400 the better headphone to me, but that's because I prefer a balanced sound, and the PRO900 is so unbalanced and wonky with it's mega-mid-bass and mid-hole with super-treble that it's hard to really enjoy music on. The A900X is like the SRH940, but with bass response. I consider the A900X superior to the HFI2400, and vastly better as a complete headphone than the PRO900. I think at this point, if you wanted something open air, I would explore the Hifiman HE-400 and/or Beyer DT880.
Very best,