I see this as a band aid, instead of a real fix. I would replace your monitor(s) with one that covers the entire range, including low end, sufficiently.
How far down do you tune your guitar?
Hi,
I have the KRK10s combined with Rockit 6 monitors.
I have mixed feelings about my whole speakers decision but I would say they are not really what I was expecting.
Basically I bought these before I became a little more knowledgable in what I'm searching in terms of sound but anyway...
The 10s is very loud. Way too much for the poor Rockit 6. So loud in fact that everything just becomes mud if you are not careful and crank it too much. I was never able to find a good spot for it where the bass would sound fine where i stand while playing. I find that the volume on it is temperamental. At low volume of the main setup, I have to reach and turn it up a little while if I crank the main volume of the whole setup, I have to lower the 10s... It's annoying and I smack my head under the metal braces of the table where it is located almost every time...
Just to give you an idea how loud it is, the 10s volume is between 1/8 and 1/4 of its range and it becomes very nasty when I crank the main setup volume. It's way too loud for a large basement where it is setup. Somewhere else like outside or a stage, I don't know...
I tried to set it up using an audio meter so everything was equal between the monitors and the sub but when I followed KRK recommendations on this, the sound was really bad and the bass was ridiculously loud and muddy sounding. My wife thought the house would come down and of course I was blasting it on purpose because it was funny but that only lasted a little while. 
Anyway, your mileage may vary... Now that I know a little better, I think the 10s is good to crack the studs of your house, break concrete or even your sternum if you feel like it. Also pretty good for dance, techno and rap music but not so much for guitar playing even with a backing track.
My 2 cents. Cheers,
Marc.
