Boston 746s have been the only 4x6 that I've used for nearly 10 years. They aren't great, but they're lightyears ahead of any other 4x6 I've heard with the exception of the Infinity plates that they had a few years back. I don't know whether Infinity still makes those plates, but Boston is making the 746s again.
The 746s have a fairly natural sounding midrange, and tolerable treble. I wish they used a real tweeter instead of that mylar piece of crap, but I will say that it's the most natural sounding mylar tweeter that I've heard. The bass doesn't exist, and these speakers are basically useless below 150hz, but then again, so is just about any other 4x6. These speakers seem to be pretty durable though. I've been abusing one pair for 10 years, and back in my booming days, the mids would bottom out constantly, but still kept on chugging. | 
