Would like to know if fitting amp to stock ba stereo will make much difference?
If so What sort of amp????????
What if I wanted to put a sub in boot,which amp would be a good suit for both applications?

BA's have a shocking head unit with no pre out puts (normal conection to external amps). Some amps do have a speaker level input that can be used, this is a second best type option but it's all you can do in a BA. I worked on a BA ute done this way, higher quality front speakers and a 12" sub running of an amp conected to the rear speaker out put. It gave a vastly improved sound to standard. Any good audio store could advise on this.

i run a cadence 4 chanel amp and kenwood 12 inch sub, running through the battery, using an aux cable to run my iriver, sounds beautiful