Turn your speed volume adjust all the way off. That's the best you can do. Other than that, buy and amp. OR you can go with the JBL MS-8 as some have. But you'd have to be serious about sound for that one. lol
That's the nature of sound; it's logarithmic, meaning that increasing power linearly results on non-linear volume increase. Low-end (bass) requires much more power to drive than mids and highs, so it is the most noticeable. Some high-end stereos do a good job of boosting the bass as you increase volume to keep the EQ consistent.
Ever notice how on pretty much every stereo, turning the volume up past 1/2 or 2/3 doesn't do much anymore?