So I just recently purchased the krk rokit 8 studio monitors for many reasons. I am pleased with them but playing with the idea about getting a sub to compliment them. In our groups humble opinion (there are many online) what benefits are there to getting a sub and if I do decide to get one should i stick with the krk 8 sub or mix it up? Thank you

This would most likely depend on the the size of your listening space, the style of music you are trying to create and  - what kind of acoustic treatment you have or plan to install. Most importantly - Are you using your monitors for critical mixing mastering type of work? or are you more into having a kick ass jam room ? Subs are great for that. They will only fill out the spectrum in the 30 hz to 40 hz zone when set-up for critical monitoring and when used with decent sized / quality monitors. I generally roll off at about 40 hz for the adult contemp / R&B / jazz stuff I do.  I happen to have a KRK 10S sub sitting in a box unused. PM if interested.

my buddy just purchased some nice HEDD type 7 monitors..
7" drivers, small room.
but he's mixing a club mix, and when he pushed a 40 hz signal from a synth,
it made his woofers jump like crazy, and made his two ports on front actually distort.....
manufacturer says it's a design limitation of the speaker, and that if he need lower frequencies than the specs call for, AT MIXING VOLUME, he'll need a sub.
 
 
he's looking at the KRK and jbl offerings
 
https://www.amazon.com/JBL-LSR310S-Powered-Studio-Subwoofer/dp/B00I08RT3U?tag=techsounded-20
https://www.amazon.com/Yamaha-HS8S-HS8-Studio-Subwoofer/dp/B00DHSK8C2?tag=techsounded-20
https://www.amazon.com/PreSonus-Temblor-T10-Powered-Subwoofer/dp/B00GRDDM5E?tag=techsounded-20
https://www.amazon.com/KRK-Powered-Studio-Monitor-Subwoofer/dp/B01I46H4AK?tag=techsounded-20
 
 

I now have 3 subs I picked up at Goodwill all for under $30 each.
2 are Yamaha's and one is a JBL. 
They all go Boom , Boom 
I took the bigger Yamaha to a gig once and had to turn it down a bit. Was a small room but still I didn't expect it to push out that much sub bass. I used to drag around a 18" powered sub. Won't fit in the Subaru.