My favorite for a midbass 8" is the Boston Acoustics G108. It is intended to be a sub but is able to play the higher frequencies. I am currently building a system in my personal vehicle with these as mid bass drivers in the front doors.
I feel I am lacking volume at the end of the rope for the rider. I have bass in the boat and 6 -6 1/2 two way speakers in the boat. the tower has skylon tower speakers with one 6 1/6 that works good and a 8" bass (or what ever it is) speaker that I can't hear very well. I would like to find something that I could hear at the end of the rope. I was hoping to find a speaker that would be very easy to put in with very little mod if at all.
OK- Do you feel you are lacking volume in the lower midrange - mid bass area (kick drums or lower end of a males voice)? This is the only area that is going to be affected by changing the 8". What Skylon model do you have and how many? What amp are you using to power them? Lets see if can narrow down the results you are looking for before you replace the kicker.