I have a kraken and occasionally still use it for indoor games. I have used both 9.6v and 8.4v batteries without a problem, however with the weaker spring I installed in mine the performance difference is almost unnoticeable. With the stronger stock spring you will notice a slight difference in performance especially when it comes to ROF. HOWEVER this does not mean you should use a 9.6v battery in a gun such as the Kraken as they are just lower end chinese clones and will be much more likely to break under the stress of a higher rate of fire. The only reason I risk using the 9.6 is because thats the only voltage of battery I have ever needed four my more powerful outdoor AEG and the Kraken is nearly a year old now and is really beaten up anyways. So in summary if the Kraken is your baby and you can not fathom doing any thing that may hurt it go with 8.4v. If you want to go up to a 9.6v be ready to do some fix ups down the road like replacing some weaker points in the gearbox such as the plastic bushings, the plastic spring guide, the cheap plastic tappet plat, or even the cheap gears or piston (I am not sure if its plastic or nylon fiber).