Point 1, absolutely. I felt the M1060 required an amp already and definitely have to turn the HE4XX up more from the same amp. Not a ton, but more for sure.
Already mentioned my solution to 2. M1060 cable is much better for my use. For those curious about alternatives, you don't need a TRS plug, and I have no idea why Hifiman is cheap enough to do it this way (3 pin TRS jack in the headphones with the ring tab disconnected and the cables are the same way at the plug) in even their expensive stuff. Even Monoprice's goofy crammed in jack is only TS as required.
3. Clamp was awful on mine. I bent them out and it's better, but I don't like the headband design. The M1060 band is extremely cheap feeling, and the drivers swing around way too freely, but I think it's still a bit more comfortable. Outside of the K7XX, all of these have steel bands that can be tweaked to varying degrees to make them better in regard to clamping though, so that's fairly minor.
4. The grille cloth on one side of mine has a weird little snag in the grille hole, but super minor. I've probably pushed more power than your mobo audio is capable of through mine and had no rattle, so I'll back that up.
5. More power will help for sure. Mine sound pretty dead until they get enough power to them. Still, I ripped my M1060 pads off and sandwiched them between the HE4XX drivers and my head and they sounded better than anything I got from them in stock form, at least to my ears. Not saying they were ideal, as they're oversized, the positioning was odd, and they weren't actually attached to anything, but the amount it did change had me looking to alternatives already. I don't know that I'd stick with the solid protein leather with a swap, but just being significantly thicker seemed to help some of my complaints. That said, I'm not a fan of the velour, so maybe I would try a leather pad just to get some of the itchy velour (Beyerdynamic DT250 pads and Sennheiser HD 25-1 II pads don't itch for me, so I don't know why these bother me, but they do) out of there.
Overall, good, but there's plenty in competition at this price point. Definitely worth checking out everybody's opinions on these and the others before jumping on one (unless you just want all of them...).