In my secondary system, which is a five speaker AVR/TV system, my five speakers consist of three NHT Super Zero 2.1 speakers and two small wall mount Definitive Technology speakers. I have had them for quite a while. When I listen to two channel music, the NHT Super Zero's are the stereo speakers. The Super Zero's are little acoustic suspension speakers with 4.5 inch woofers, which I have mounted on the wall Those speakers now cost about $250 a pair.
My first sub woofer in this system was an inexpensive Pioneer SW-8MK2 that recently died. I replaced that with an SVS 3000 Micro, which cost $899. Once that I got the SVS 3000 Micro dialed in, the improvement in sound quality was astounding for both 2 channel listening. It was like listening to a different set up speakers. Changing the crossover to 120Hz instead of 80Hz made a big difference as did fiddling with correction settings on the SVS sub woofer.
The sub woofer cost to speaker cost ratio in this setup is extreme, but it shows how much difference a good sub woofer can make with smaller speakers. With the SVS sub woofer, 2 channel music in this modest system comes much closer to the sound of my much more expensive primary system.