what are you looking for in a soundsystem?
quality? loudness? lots of bass? surround sound? small form factor? etcetera.
you never listed what actual equipment you have. brands and model?
i'll give you two scenarios of what i personally use.
in a room about 12x14 i have a receiver, 5.0 set of satelite speakers at 50w, 200w peak with a center 75w 300w peak and a 450w subwoofer.
sound quality is good and volume is more than good enough. i use it for gaming, movies and rarely movies.
in an open room about 15x20 i have an older tv with 2.0 set of floor standing speakers at 150w
sound quality is decent and volume is amazing considering they arent ran through a dedicated amp (connect up to an old tube tv). honestly for $100 a pair they were a great buy. this tv is used for tv/dvd only and quite frankly 2.0 is fine for this.
the first set cost $1300 total, the second set cost $100. the more expensive set sounds nicer but both the first set (with sub) and second set (no sub) can both produce good volume and bass.
paired speakers may be the best option for quality. just adding speakers of any brand can sometimes be detrimental instead of an improvement.
some large tower speakers do not require a subwoofer. in most cases one subwoofer should be more than sufficient. i have heard of 2 subwoofer units but unless you want extreme amounts of bass and have an amp/receiver which can handle it then it may not be worth your while
at 300w your speakers may not be able to be driven by just any receiver or amplifier. my $500 unit for example only offers 80w per channel.
throwing more speakers and subs at an issue isnt what you should be doing. you need to figure out what you want your system to do first.