Obviously, you've got decent earphones, so you should hear great sounds. As denislawsbackheal says, you should try another source which will identify if the problem lies with the earphones or the source. Earphones by Sennheiser probably come with 2 or three speakers in each ear, so unless you've driven them beyond their limit, then one of the bass speakers may have failed in either the left or right ear. You should be able to hear this imbalance without too much trouble using a decent source. I think it would be unlikely that you've blown both bass speakers at the same time so the source would seem to be the problem.
Finally, if you have blown your earphones, it is possible to get them repaired rather than fork out for new ones, depending on their value. A google search will lead you in the right direction.