As a quick follow-up... I upgrade my speakers to Polk Audio DB651's last week in preparation for our first trip of the year. The wires have clips for the terminals, so connecting is fairly easy. I didn't need to use the spacers that came with the speakers, just drilled new holes and used the screws that came with the new speakers.
On word of caution - check the speaker wired for polarity, as on one speaker the black was negative and on the other is was positive. I honestly can't tell the difference in sound based on the polarity but I wanted to get it 'right'.
The new speakers sound so much better!
No idea on a sub-woofer. I wouldn't know where to put it and would be too worried about making my camp-neighbors angry with the noise.
2015 Thor Outlaw w/ FJ Cruiser Toad
Former owner - 2013 Jayco X20E