I have just bought a 2nd hand sub in a home made box, and i am
currently powering it with a 300w jet sound amp.  Not sure of the rms
of this.  Althought some sound is coming out of the sub, i don't think
it is as loud or as punchy as it should be.  Could this be due to the
lack of power supplied by the amp.

Now on to some theory.  Just wondering if say a 300w sub would be
working at full output at 300w power?  Is the jbl 100w only working at
1/3 power with a 300w amp?  Therefore with a 300w amp, better and
louder effect of the sub is achieved with a 300w sub rather than a
1000w sub.

Hope the explanation was clear but any help as to how subs and amps
work would be greatly appreciated.  Thanks in advance guys.