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When you look at many of the pictures of setups on this forum,you will notice that systems contain multiple amps,because they do different things well.A modest output low distortion, 4-channel AB type amp may be powering the front speakers,while a high output class D mono amp or amps(which typically runs cooler but has more distortion)may be powering the rear subs,for example.Also six or eight channel amps are available,where pairs of outputs can be bridged together to"up" the output for the subs.Many times physical size of the amp may be a consideration if you wish to stash it under a seat rather than in a custom rack.Crutchfields has an excellent treatise on amps and their applications on their website.
You can get into a lot of debate here. Generally speaking from a SQ/Decibel output standpoint (clean,clear,loud) you are better off having 150w RMS per channel than 45w RMS per channel as the amp is going to be less likely to be driven into clipping (distortion) and has more "headroom" to handle transients (loud transitions in music) more effectively without issue (read distortion).
Simply put, with proper set up a 150w amp will play louder (and cleaner generally) than a 45w amp ever will. You have to take into consideration the driver type you are using and its recommended power rating. IE 6.5" component speakers with a 100w RMS rating would play/sound better from the 150w amp than it would with the 45w amp keeping all settings the same and the amps are of equal quality.
IMO with amps you get what you pay for ... spend $150 on a 4x 50 watt amp and you will be lucky to get 4x20 clean watts. Spend $350 on the same spec and you are likely to get 4x50 or even 4x65 clean, useable watts.
You can get into a lot of debate here. Generally speaking from a SQ/Decibel output standpoint (clean,clear,loud) you are better off having 150w RMS per channel than 45w RMS per channel as the amp is going to be less likely to be driven into clipping (distortion) and has more "headroom" to handle transients (loud transitions in music) more effectively without issue (read distortion).
Simply put, with proper set up a 150w amp will play louder than a 45w amp ever will. You have to take into consideration the driver type you are using and its recommended power rating. IE 6.5" component speakers with a 100w RMS rating would play/sound better from the 150w amp than it would with the 45w amp keeping all settings the same and the amps are of equal quality.
IMO with amps you get what you pay for ... spend $150 on a 4x 50 watt amp and you will be lucky to get 4x20 clean watts. Spend $350 on the same spec and you are likely to get 4x50 or even 4x65 clean, useable watts.