JL amplifiers are very nice. I have never used the 300/4, but I have a couple of 300/2s right now. I think they are great SQ amps, but I doubt they are the best. Best for that price range maybe, but if you really want the best, I would probably look at brands like Zapco, Sinfoni, Tru Technology, etc. But those may cost well over $1000 for an equivilent amp.
I doubt you'll find a better amp for the money. They were designed by the same guy that designed the PPI Art series, if that gives you any clue. It has everything you'd ever need out of an amp.