Hey pal, well when it comes to car audio, I have done alot of rescreach and I have used a number of brands.
IMHO Vibe are "ok" but only if you buy the £300+ setups.
In terms of what size subs, you need to look at the type of music he likes, for example if he like dance/trace he would be better off with 10's as they punch better, R&B id go for 12's. As for 15's id stay away, two 12's/10's will do a better job imho.
When it comes to AMPs
ALWAYS use a monoblock amp for subs, it will cost more but they are made to power subs and only subs. They carry a much better signal and have a less problems with underpowering subs and blowing them.
Now the next thing you want to do is look at RMS power are what Ohms the subs run at and what Ohms the amp can power for example.
I currently have 2x 10'' Infinity subs, they are 1300w each but are running about 250w RMS power and will run as low as 2 Ohms. (means they can produce lower bass freq).
Now for my amp I have an Alpine MRP monoblock (Brilliant amps) that amp will push out 500w RMS and run 2 Ohms.
SO I have my subs bridged on the amp meaning they will run at 2 Ohms, since having this setup my amp has been turned to min bass level as there so bassy if I turn it up lol, my bass power is then altered via my H/U.
Hope this helps
