I did go too far because I cant re-sell this.
I did the HD800 driver implant to HD600 3DP enclosure but I found the HD800 driver to sound a little piercing and was unable to tame. Some say HD800 driver has a resonance in treble area but I dont know - just my anecdotal impressions from testing and dubious correlation to some dubious graphs. I imagine HD800 driver frequency response plays better with pads with a deeper and larger ear chamber for more v-shape tuning which would mitigate the lower treble peaks / sharpness, ya know like stock enclosure…
I havent listened to HD650 dumb mod since. Not because its bad but Ive just been busy with other stuff. As for headphone activities, afterwards I modded HD600 and am currently modding a Stax SRD-7 (replacing resistors) and SR-X mk3 (padrolling). And this week Ill be installing vintage ortho drivers into an open-back 3DP enclosure. I dont have as much interest for headphone experimenting as I once did so going is slow.
My next mod I want try with 6X0 is using supraaural foam pads like Grado. If there seems to be great potential then Ill see about 3DPrinting some Grado style enclosures for 6X0 drivers. The design would be a simple driver drop-in like the stock capsules and using stock headband. I wonder how 6X0 with foam pads compares to Grado RS-2x with same pads (“flats”).