I have DT770's for nearly 10 years now, and I'm not going to change those headphones anytime soon, because I believe they are perfect ones for my needs, and I'm used very much to their sound. This is my first time to change the ear pads after many years of almost everyday use, and many washing cycles, the old ones got too soft and squishy, and the headphones where pressing directly on my ears, which resulted in decreased comfort during long studio sessions, and false sound picture.
So I'm very pleased with installing the new pads, and there was clearly noticeable positive change the between the way they sounded before, and how they sound now.
So make sure you change them as often as possible, or if you feel they are getting worn out.
Don't be like me and change your ear pads much often, it is one of those little things that you don't realize the difference they make, until you replace them with new.
They come in pair with the spongy membranes that sit over the drivers itself.