I got another view that I am sure will stir up some disagreement:
Put the $170 into an upgraded alternator rather then a stiffening cap. You will not be able to tell much difference in sound from the cap. Newer amplifiers have plenty of capacitance built in to handle their rated output power. If the cap helps noticeably, you have too small gauge power cable or too small of a battery and/or alternator. The headlights dim from the alternator voltage dropping because the current draw is higher then the alternators output capability, so the voltage drops down to the battery's voltage (~14v down to ~12v). The cap will help a little but it still charges from the alternator so once the cap is discharged (which happens quickly) the lights will still dim. Also, going with an upgraded battery can help. Optima batteries have more capacitance then a standard battery so they charge quicker and then can also handle more load with less voltage drop then a standard battery.