My definition of bass quality is how fine the bass is; how punchy, how "controlled". Some headphones have big, boomy, overpowering bass. This is considered to be the lower quality bass. But it depends on preference. If you are the type that likes to listen to the bassline of songs, then you would prefer boomy bass. If you want a 'balanced' sound, then you should go for headphones that give quality bass, instead of quantity. If you get a chance, try the XB-series. I thought the XB300 was good enough for bass, they certainly have more bass than my HD438s. They're not too bad considering their price.
If I were you I'd look for something in the XB-line.
Take my comments with a grain of salt ya.