Burn-in is a documented fact for large dynamic transducers - but the effect is very small.

The best way to "burn in" a headphone is by listening to it - because it is your mind that adjusts to a particular frequency response. More often than not when something has "no bass" it's because you are used to "too much".

I wouldn't give too much thought about it and start listening to your headphones more and see if you can adapt to lack of "OMGAWD BAZZ" as there are very few decent headphones with a truckload (at a stretch the DT770 and DT990, some Ultrasone models and XB series, none of which I personally like or would recommend)

I'll get back to you muffindo, weather is too shit to think about it. Feel free to check out Sony XB700 if you want huge bass. I'd recommend trying to get a used grado/alessandro model or something or just sticking to a KSC75