Nice setup, hope you make another post when the tubes come in.

Sidegrading to the 6XX isn't worthwhile in my opinion, 580s are great as is so it'd be better just going for a straight upgrade or sticking with them.

You won't destroy the headband if you bend ONLY the metal, don't worry about it. Do it as shown in this video (but since you want it tighter just do the bending the opposite way).

I am not sure about the earpads the Drop edition of Sennheiser headphones use honestly... however I do believe the ones in the 500 series are different than the 600 ones but don't take my word for it.

You can bend the metal part of the headband inward to regain some of the clamping force back. There are some tutorials for this online if you need a visual example, it is simple to do.

The HD600 and other models in that line of headphones are notorious for the pads having a significant impact on sound as well: if the pads have been worn-in, sound signature is noticeably impacted so you will want to look into a pad replacement, only genuine Sennheiser ones though as third-party pads don't play well with these.