Hi Sashav,
Alot of opinions here. My experience says, if you like the sound of your smaller speakers and your amp will power them properly for the room your are in, you are fine and can achieve wonderful results with a subwoofer. You can run into a problem crossing. If you do not us an electronic crossover to cross your satellite speakers where you want them to blend with your subwoofer, you will need to cross your sub woofer around the 3 to 6db downpoint of your satellites I use a subwoofer now and cross it @ 40hz (its lowest setting). My speakers are 3db down at 38, I set the woofer low and slowly added until it was smooth, played a bit with the phase switch, had to move the woofer a couple of times, but ended up with a very smooth transistion.
As far as woofer size... on a properly designed woofer, size does not matter. A designer should have more motor to match the mass. Good 15's are fast, but you should try to listen to the sub that you want first and consider your room size, an 8, even a 10 would not move enough air for my room.
On the other hand, if you really like the larger speakers.... your done.
Alot of opinions here. My experience says, if you like the sound of your smaller speakers and your amp will power them properly for the room your are in, you are fine and can achieve wonderful results with a subwoofer. You can run into a problem crossing. If you do not us an electronic crossover to cross your satellite speakers where you want them to blend with your subwoofer, you will need to cross your sub woofer around the 3 to 6db downpoint of your satellites I use a subwoofer now and cross it @ 40hz (its lowest setting). My speakers are 3db down at 38, I set the woofer low and slowly added until it was smooth, played a bit with the phase switch, had to move the woofer a couple of times, but ended up with a very smooth transistion.
As far as woofer size... on a properly designed woofer, size does not matter. A designer should have more motor to match the mass. Good 15's are fast, but you should try to listen to the sub that you want first and consider your room size, an 8, even a 10 would not move enough air for my room.
On the other hand, if you really like the larger speakers.... your done.