I have a problem with my Imac : the sound is very bad. With the internal speakers, of course, but even with Sennheiser HD 205 headphones. I have an external Tascam sound card US-122L and the sound is not better when using it.

A piano solo piece is really really hard to listen due to bad sound, and the same file on a mac pro without sound card, or on a pc sounds very good.
I have this problem since at least 6 month.

Thank you for any reply/solution.