Audio Systems: matching components, subwoofer system, order of magnitude

Question
Hi need help with choice of 4-ch amp to use in my setup...i have 05 Sentra with tweeters in A-pillars..currently I have all Clarion setup..CZ300 head unit, SRQ1621S Components in the front, SRQ1631R on rear shelf....I also have the SUB inclosure from the rockford setup for car but will change the driver or complete box base on what you recommend....OK a little details on me..I don't like very loud music but love separation, clarity and a good clean not loud but somewhat deep bass with a good thump...I was looking at 3 class D amps..Kenwood Excelon XR-4S, the JL Audio XD400/4 and the Alpine PDX-F4...Also the freq resp. of the spks are 30-35k..does this make a difference of the unit

Answer
Well, first off the acoustic environment of automobiles is rugged.  The sound level must be much higher than in a typical home environment due  to the inherent noise in an automobile. So while it may sound not so loud, in reality it is.  To get a good bass thump you must have a subwoofer with plenty of bass capability and, if you are listening at "low" levels, the sub amp would need to be turned up more than ordinary.

Ordinarily even the best single driver mounted in the doors and rear shelf will not produce solid bass due to their mounting configurations.  So, you need the subwoofer system.  

Frequency response ratings on speakers is primarily fiction. The only way you can get bass frequencies from a speaker is to have it mounted in a special design enclosure box.  Subs have that but door mounted and shelf mounted do not.

A good amplifier will do the job if you choose the right sub.  Any good amp will be adequate; the sound quality of amplifiers is an order of magnitude superior to the reproduction of sound by a loudspeaker so the emphasis should be on the speaker selections, sensitivity and its enclosure.

There is virtually no way to predict how a speaker system will sound in an automobile because of the acoustic space variations.  Some trial and error is always necessary.  The location of the sub in a vehicle can make a great difference, for example.  Just moving its location from one side of the trunk area to another can make a really big difference.

Hope this lecture has provided you some ideas for a good solution.