I don't think just throwing MORE speakers at the problem is the most efficacious choice. It takes more power to run more speakers and can "water down" the output. To me - lots of speakers just clutters the room and muddies the sound stage unless they are being utilized in a proper surround sound arrangement. Of course, that's just my opinion. It's your ears that have to judge your system, and if you're happy then that is what counts.
Lots of people will add a sub-woofer to a speaker system to round out the bottom end. Some people add super-tweeters to enhance the high end. Some people modify speakers by swapping out different drivers or tweaking the electronic crossovers. Some people just look for "better" speakers to buy.