I'm not that much into car music but have dabbled a little.

Is he looking for bass? if its just just a general improvement over the often poor radios car makers put in, then replacing existing head unit with say a Pioneer 4 x50w RMS per channel will see a massive difference, however once fitted you might find the standard speakers can't cope, so you get into swapping those out too.

The amps you link to are mainly for providing power to a sub woofer, not only do you need the amp you also need the heavy speaker in its heavy and substantial box, by the time you've wired (direct feed from battery to amp/speaker, and signal wriring from head unit to speaker) and fitted the amp half or more of his boot will have vanished.

Active subs come with a built in amplifier and thse are the easiest to swap about IMHO, various makes, i had a fairly cheap and cheerful but really quite decent Fli Trap which is a reasonably compact 1000w active sub in my W124 Merc (that model MB comes with stupidly small fitted speakers so to get any decent bass without chopping the interior up a sub was the best bet for me.

Anyway, if he wants to keep the boot as a boot then Pioneer make a quite decent active sub that fits under the seat in most cars, obviously it isn't going to shake the foundations like a 2000w twin 15" speakered remote amped jobbie is.

Daughters Aygo came with a poor standard head unit and hopeless speakers as standard, could barely be heard over the road noise, for her birthday several years ago i swapped out the head unit for a Sony 4 x50w, swapped out the spectacularly awful speakers for Alpines (tried the standards with the Sony unit, oh dear) and fitted one of those Pioneer under seat active subs under the passenger seat, the results were excellent they are really pleased and left the already small boot intact.

All depends what your lad wants to achieve, there's loads of decent info online and you don't need spend a fortune to get good quality sounds, however being a young man with probable peer pressure to conform (and like it or not thats a big influence these days on many people of all ages) there might be certain makes and labels only to choose from..:-)

Don't know if that waffle helps at all.

As for makes, i've had good results and reliable service from Pioneer products for many years now.

Whilst high street shops might be good for info etc, they charge well over the odds for wiring and fascia adaptors etc, so read up compile your requirements and do some shopping about, you can easily save more than 50% doing so.

As GB said apart from mentioning RMS output, some units, ie, speakers and amps mention outputs as peak music output which is misleading so wherever possible look for rms, ie could be 50 watts rms which is better than 50 watts music power or pmpo as some put it, but the rms is more powerfull and cleaner sound usually. I hope I havent confused..

(A lot of these aren't even playing music, it's usually this awful "rap")

Dont know how you can tell, its usually so loud and distorted it could be anything

some of it is competition between mates, who has the loudest/noisiest sounds in the car, including who can screem the engine the loudest