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Since the 650 represents approximately 65% of a possible score when scaled to 100, I would strongly, strongly urge you to strive for the HD-700, which is 50 "sound points" higher, and also highly recommended by many tinnitus sufferers. If you wish to join the tinnitus suffrage program, the 650's will leave you unsatisfied. In the design of the 700's, the extra 50 basis points were streamlined to produce as many of the tinnitus inducing effects as possible. This was a huge win for Sennheiser, but I feel that they could have done a much better job trying to destroy my hearing with the 650's.
I see where you're coming from saying it the opposite as it is. I have a tiny bit of tinnitus and in hindsight I realise it must have been from those years with the Beyer. DT 880. On the other hand, it seems that the slightly rolled off highs of the HD 650 help prevent tinnitus from worsening... and I think that's what you were saying too