KRK S10G4

I have stellar Neumann KH750 to compare and without a doubt I can say that KRK despite being quarter of it's price is a very good sub. Smooth and powerful with a coherent sound that mixes well with a monitor. Unlike other subs in this price range it plays down to about 30 Hz and is free from farting harmonics about 100-200 Hz (stay away from Presonus Temblor for this reason). In one of my setups I connected it with JBL 308p v.2 and it is a killer. Generally it is worth to add sub to any setup for two reasons. Firs it gives you one instead of two source of bass frequencies that you can place in a room whenever you want. Thus minimizing standing waves in a bass region. Second when you take low end from you monitors "to the sub" they just play better.