[Rob80b]

This is has been confusing

Jermo uses capsules and and drivers interchangeably in the same paragraph just as the capsules/drivers can be interchanged.

Just to add some background: An HD 660S2 with the HD 650 capsules (drivers) will sound exactly like an HD 650, since the earpads and the baffle (metal mesh part around the driver) is exactly the same. The same applies vice versa.
You can get a different result by implanting HD 660S2 drivers (capsules) into the HD 600, since the baffle has less acoustic impedance and you lose some amplitude below 1kHz, which makes the upper mids more forward and less "mellow".
That being said, I don't endorse taking your headphones apart ;^)