I've had a set of these for about 4 years now, but the right speaker recently stopped working.

I bought a replacement set today but my issue now is that the new ones don't sound anything like the old set. The high's and low's are strong but the whole mid range sounds quite muffled.

Will this change over time?  I've seen people talking online about a burn-in period, but I'm not sure if that will do the trick.

Advice is appreciated, thanks.