Intake Filter Question

[Growin Grass]

As it starts warming up I realize my new setup needs a little more air flow, I'm currently running a 6" passive intake with a merv 11 furnace filter, I already plan on bumping it up to an 8" but I was wondering if I went overkill on the intake filter would a merv 9 do the trick? I live in area prone to PM so making sure I've got clean intake is important (I know, I'll probably just wind up tracking it in anyway)
Thanks Farm...
Edit: I'd really like to keep it a passive intake if I can

 

[3N1GM4]

1:4 intake:exhaust ratio to give room negative pressure
i.e. 100cfm intake fan, 400 cfm exhaust
Or if your fans have controllers you just speed up or slow down one fan until you achieve good negative pressure