Will these subwoofer be enough ?

I have mc tube pre amp and set of 601 mono blocks with new klipshorns mk6 system sounds good but seems to be missing low end on certain music. Thought I would add a sub and then read here that with the mono blocks two subs would be better.than one .    I like REL as a brand and their classic line will look the part next to my horns . Ax 4k for two of these 12” . That’s about limit of my budget for a sub.  Will two of these smaller subs have the impact I am hoping for or would one 15” REL carbon or similar be better.? Is it highly recommended to have two subs with monoblocks ? 
recently added a Cambridge exn 100 and had  to immediately turn down treble . I have gotten used to it and works perfectly compared to troublesome McIntosh mb50 I had but seems there could be substantial improvement in sound there.  
 

any suggestions would be appreciated. Can buy subs from crutchfield and send back within 60 days.  May do that with the Cambridge unit as well.  
 

thanks everyone enjoy this forum