If you've got US$500 to spend, check out the new KRK RP8s.
To my ear, their bass is tighter and more accurate than the Alesis actives. The Alesis monitors do "sound better" to some people, because of a hump in the low-frequency response which helps them sound "bigger". But remember, you're not going for "good-sounding", you're going for something that'll reveal the flaws.
The KRKs are slot-ported, reducing the "chuffing" port noise when cranked up, which round-ported monitors tend to exhibit. The woofer is glass-composite rather than polypropylene, which sounds better (tighter) to my ears, and less resonant.
That said, what's really most important is that you learn your monitors. Listen to CDs with which you're intimately familiar, and to which you've listened on all sorts of playback systems, under all conditions.
Bring those CDs to your local retailer and check out all these monitors head-to-head. The important thing is what you hear, and with which you can get your best work done.
-Hoax