With the tweaks, the HD650 sounds faster, crisper, more lively, less over damped. I would not attempt these tweaks on the HD600 though. Overall, maybe a few db too much mid-bass, but everything else sounds just right.

I removed the rear foam and heard the mid-bass as a bit too boomy/muddy, similar to what I hear with a higher Zout source but less pleasant (dual mono Buffalo IIs directly into a Lundahl amorphous core OT).  I wonder if there is a subjective difference between decreasing the acoustic damping (the mod) v decreased electrical damping (higher Zout).  I also wonder if the "quarter mod" would re-store the subjective balance (I replaced the foam and didn't proceed with this).  Did the CSD change with the mods (I realize it might not have been measured pre- and post mods)?

CSD should show longer decay, since there the air is now more compliant and does not as strongly resist the driver's excursion and return to the center position. With treble, this is good, since the treble wasn't all too present stock. I assume mids sound more wet/full/lively (like a very brief echo/resonance, much like the one you hear in your chest when singing). But the bass is becomes slower than it already is (although now at a lower dB). I think that this mod is a matter of exchanging better FR and impulse response for worse CSD. FR is becomes flatter because the back foam is a semi closed chamber that reinforces bass at a certain frequency and attenuates but causes some ridges in treble (since the foam reflects some treble back). Keeping front damping without the quarter mod also contributes to bass resonance and treble attenuation (and spiking).

I see how you describe the headphones as boomy; the modded HD650's articulation is faster but less controlled.

I Feel like the HD800 + HD650 could be my end-game and would surely cover all bases for my music enjoyment.

I Feel like the HD800 + HD650 could be my end-game and would surely cover all bases for my music enjoyment.

Exactly my thoughts too!