As the pad gets worn, it’s smaller.

How that changes sound:

Your ears are much more closer to the driver. Probably even for a half of centimeter. Mids get much louder, you turn down volume, the bass and treble lose their impact.

The sound signature ends up sounding muffled and shouty.

Because of lower treble and closer drivers to the ear, the soundstage also becomes flatter/in-your-face.

Quick test /proof: listen to the headphones, and just pull each earcup a tiny bit further away from your ears, immediately they’ll sound more spacious.

But the bass won’t improve since also the bass vibrations are lost on the old pads irretrievably.

So get the new pads every year or so or you’ll start hating your HD600.