If you want a relatively high end amplifier that's also affordable, consider the Sounderstream Reference amplifiers. These use very good internal components, but they are Class A/B or Class A/B related, so they're pretty big. If you want a high end Class D amplifier, consider Alpine PDX F4/F6 or JL HD. If you want an affordable decent Class D amp, see Alpine MRX-F65. If you want something very cheap, perhaps a PPI Black Ice 4 channel (there are several models).
JBL MS A1004 seems like a nice little amplifier, but a little overpriced for what you get. The main "enhancement" relative to competition is that it has some limited DSP capability built into (active crossover, time alignment, and maybe some other things.. I'd prefer to get a Pioneer DEH-80PRS + a random 4-channel amplifier for the same price).