I've never heard the L300, but I do have 4430s, and I've read many threads where the comparison was discussed. Bear in mind that they have identical cabinet volumes, and the woofers and compression drivers have a great deal in common.
Starting at the bottom, one comment I have seen is that the L300 has tighter bass, with the 4430 being "tubbier." (I'm not sure exactly what that means.) (On my 4430s I have
tuned the ports down a few Hz--something Zilch suggested--and
raised the speakers up on stands--another common recommendation--and I really like the bass.)*
As to the midrange, I have heard that the L300's aluminum diaphragms sound a little smoother than the Ti phragms found in most 4430s (2425/2426 drivers; the very early ones (2421s) were AL). I have read other threads where people found no great difference between the two phragms. (In any event, I believe that the
$80 AL Radian diaphragms for the LE85 / 2421 / 2425 / 2426 drivers would address this issue nicely, while their Mylar surrounds would afford more power handling and longevity than the earlier AL phragms.)
And up top, having those sweet 077 tweeters, the L300 will play up and past 20 KHz, but the 4430 rolls off at 16 KHz. (Long after my hearing rolls off.)
Overall, the 4430 has more uniform (100 x 100) dispersion (I believe).
And I have read that the 4430 has better imaging.
Another issue is pricing; I think you can still find some decent 4430s for around $1,300. Clean L300s are fetching $4,000-5,000 nowadays.
* One thing I highly recommend on the 4430s is
bypassing the biamp switch; that greatly improved the bass on mine.