I think that either the speakers or the amplifiers may be bright sounding, however I wouldn't start by replacing them. An overly simple solution might be to fit a small piece of foam over the tweeter, one where you could add a piece of cloth over it to increase it's ability to reduce the amplification.
Not pretty but not expensive and not permanent and adjustable to your liking.
Not pretty but not expensive and not permanent and adjustable to your liking.