I have to disagree with your assessment that the HD650s have a more extended treble than the LCD-2s. I own both, and can say with utmost certainty that the LCD-2 is clearly superior in almost all aspects to the HD650; treble extension, transparency, detail retrieval, bass extension (as you noted), and just overall fluidity to the sound. The only area where I can say that HD650s may beat the LCD-2s is soundstage width. Are you using rev 1 or rev 2? If you're on rev 1, you may want to consider upgrading to rev 2.
Wow, our opinions are 180 degrees out of phase, but okay... what kind of music do you usually listen to? (for me, acoustic classical)
I think I have rev 2 of the LCD-2, according to my serial number (531...)
We could agree in one sense. For a moment consider the distinction I make between "bright" and "extended." Extended means you are getting real information in the upper octave; that you are getting sound that enhances the colors of the tonal palette. Something that is not extended can still be "bright" by having a peak somewhere.
Okay, so to me the HD650 are brighter than the LCD-2. If I wrote earlier they are more "extended" I'm not sure about that-- I'm not sure they are really giving me any useful information in the top octave. But no doubt they are brighter. I don't know how anyone could hear it otherwise.
The LCD-2 is a dark sound that is missing the top octave, mostly, something confirmed by FR plots but also the ears of a number of my friends who are more into speakers and have a good ear for tonal balance.
The HE-500 stomp all over the HD650 and in fact, for me, beat the LCD-2 in overall performance (although LCD-2 wins in bass performance). The HD650 just has a rough transient sound, a lack of definition in leading edges -- once you hear that definition in planar drivers like the LCD-2 and Hifiman, there is no going back. The HE-500 is a tradeoff with the LCD-2---missing the LCD-2's bottom octave but possessing a nice top octave. It presents transients differently. At the moment I prefer the HE-500 but I'm not certain -- I need to give it more time -- it may be a slight overemphasis, not sure.
Oh, the HE-500 has more precise and expansive imaging than the LCD-2, which may be mostly due to its upper-range extension (a lot of those placement cues come in the top octave). And I'm not even using with my favorite cable right now.