Well I am switching to Radian due to positive experience made by some people I know.
The sound of the 2426 basically I like less than the sound of the VHF100.
If to use Ferrofluid, how to apply and how much per speaker ?
There are multiple grades of ferrofluid available, with different viscosities that are better suited to LF, MF, and HF drivers. (Community is one of the few companies I can think of that ever made ferrofluid-cooled LF drivers.) To get the same performance, you'd need to use the right grade of ferrofluid for the speaker. While the original JBL driver probably did not use ferrofluid, it is possible that Community disassembled the stock drivers and added ferrofluid to the magnetic gap, and then designed the voicing of the speaker with the driver that had the ferrofluid added.
Look in the gap and see if you can see any of the brown liquid left. If so, you're probably all right to just install the new diaphragm and let that work with the remaining ferrofluid.