HU is fairly easy to change, don't listen to the dealers :thumbsdown:
Upgrading HU improves things greatly
Upgrading speakers improves things a little more
If you are going to go to the trouble of changing speakers I would recommend getting a decent amp to power them as the Bose one is a bit pants...
Sub & amp is pretty pants too...
So do what I have done
New HU, new speakers, new sub & new amps you would be looking at £1,500+ though
Oh and