KRK10s are subs you own? The brand/model of speakers don't have a lot to do with sub choice.
How big is your room (and areas open to that room)? What are your spl and extension goals (how loud do you need it to be able to play and what's the low frequency point you'd like the sub to be able to do at that level)? Do you require an even response over a large area or just at your listening position? What's your budget?
Hi whoa lots to think about thanks, yes I currently own the KRK 10s
1- room is approx 20'x20' with right side open to front entry
2- Using an analog SPL to 85db sub is calibrated to mains set at 75db at the 10:00 (-6db) gain position, set the gain at 12:00 (0db) pins the SPL, anything above that and it's plenty loud seating position is 14' away.
3- Iv'e read the KRK actually handles a requency responce of 30Hz well, not really sure if that's sufficent though or if it's ok.
4-KRK 10s Technical Specifications
•Configuration:Front loaded, vented subwoofer
•System type:Active Subwoofer
•Low-Frequency:10″ glass aramid composite woofer
•Frequency Response: 34Hz – 50Hz to 130Hz Variable
•Max Peak SPL:113 dB
•Amplifier Class:Class A-B
•Power Output:150 Watts Continuous @ 1% THD+N @ 100Hz
•SNR (signal to Noise Ratio):98dB
•Amplifier Distortion (THD+N): ≤ 0.1% @-3dB of Max Power Output 20Hz – 200Hz
•Input Impedance (Ohms):10 kΩ Balanced or unbalanced
•Ground Lift:On / Off
•Low Pass Frequency:50Hz to 130Hz
•HPF bypass:with footswitch only
•Footswitch:Optional
•Phase:0 or 180 degrees
•System Volume:-30dB to +6dB
•Indicators

ower
•Input Connectors: •Stereo RCA for left and right, Stereo balanced TRS for left and right, stereo balanced XLR for left and right
•LFE input:N/A
•Output Connectors: •Stereo RCA for left and right, stereo balanced XLR for left and right
•AC Power Input: Selectable 110V-120V / 220V-240V
•Enclosure Construction: 15mm reinforced MDF
•Finish: black vinyl wrap
•Port Configuration: Front firing slot port
•Finish: black vinyl wrap
•Grille: Metal Mesh
•Dimensions (H x W x D): 15″ (382mm) x 14″ (355mm) x 16.06″ (408mm)
•Weight: 39 Lbs. (17.7Kg.)