I don't understand the reasoning behind the requirement that both headphones be of similar "standard" to be complimentary. The real test of a headphone is the extent to which I'm drawn into the music, not necessarily how technically proficient the transducers are. The HD650 excels in this department. It also is different enough in terms of signature that, IMO, it makes for a good complimentary headphone to the HD800.
Your standards must make listening to music such a chore - if the HD650 is so inferior that its redeeming qualities can be dismissed. I'd hate to have that ailment.
I don't think that HD650 is so far from HD800 SQ speaking and these cans are different enough to compliment themselves IME (I had a loaned pair of HD650 in order to compare it to my HD580) . When i Purchased my HD800 I really thought i would keep my 580 as compliment. but I was surprised to see that my HD800 can finally replace completely my 580. I never miss this HD580 but I still miss a forgiving (but good) can to compliment my 800 and my sig pro . I really think a HD650 is one of the more qualified candidates for this purpose.