Subwoofers to pair with ML SL3's

Looking to add a sub or two to my system.   

Currently have a Krell preamp and amp with ML SL3's.   

Considering a used Krell sub (not the $45,000 one), or one of the ML's.   either the 210 or 212 or a pair of 1600x's. 

Does the ARC really help that much?   My room acoustics are challenged.