Anthony, take care with trusting frequency response graphs. I've seen many Stax graphs for Lambda headphones included in their manuals and they are always as flat as can be in the lower mids down to deeper bass, whereas the same phones measured by hifi magazines always shows a 100 - 150 Hz peak followed by a roll off in the deeper bass region (Stereophile did a mass comparison of phones many years ago - all phones had that same basic response in the bottom end). The frequency response cited in Wikiphonia is one of the latter. I would suggest that you would be better listening rather than depending on those graphs. I hear the normal bias Sigma as rolled off at both frequency extremes (I found the treble roll off particularly sad), and they become a much better phone with a SR-404 driver implant. Not a single drawback to that venture that I've ever heard. The phones are about the same efficiency as an SR-007, if you are familiar with that model, so be prepared to turn 'em up. The original Sigmas also came with silistors to limit input into these phones, as they were sold with an SRD-7. The SRD-7 could be driven by any amplifier and some people drove them beyond mortal limits, so Stax decided to limit how hard they could be pushed to avoid driver arc welding. I hope you like the Sigmas - they certainly aren't for everyone, but they scale up very well with improved amplification. An acquaintance over here in Perth has ordered a BHSE and I really want to hear them through that. They sound great via the SRD-7 backed by a largish amplifier. In some ways, they remind me of the old Magneplanar Tympani 1D speakers I had - they like big amplifiers and wake up when pushed a bit. The Tympanis had a unique quality to them - it was quite easy to converse when listening to them. Total lack of the ubiquitous midrange peak found everywhere else. And so goes the Sigma. Vocals and piano reproduction to die for, in my opinion.
