That is simply not true. The Companion 20 Bose speakers I use with my iMac sound excellent. It is surprising how good such small speakers sound.
You've probably never heard a pair of Series IV Mach V stereo speakers I would guess but those sound amazing. i own a pair and love them.
I like my Bose AE2 headphones as well. I like them better than the Denon set they replaced which had cost more. They're a good value for the money.
Within their given price ranges I think Bose speakers are pretty hard to beat and I like their innovative space saving designs.
Lastly, consider that we are talking what comes standard with an iMac versus an upgrade as significant as a pair of Companion 20's. There is just no contest at all. They are far, far better and do not cost a lot considering, around two hundred bucks or less on sale.