A lot of folks have added amps and upgraded speakers. I see more Rockford-Fosgate or Clarion. There are very good write ups on in fairing installs for these amps. I did not want to give up bag or trunk space. I went Clarion and did a detailed write up. Einstein has a great write up on the RF. The speaker cones in the fairing do not need to be removed. I left them for weather protection and cut a couple of holes in them.
No matter what, with the 5 1/4" speakers you are not going to get great consistant lows. I heard significant bass distortion on the video that the OP linked. With 4 speakers on my voyager a lot of the lows are dependant on the song. Some Josh Turner sound great and others will distort... Mids and Hi's are awesome.
2009 Kawasaki Voyager