I was thinking that the 3/8 line itself was causing some of the restriction, but that seems not to be the case.
I agree about either bleeding off the excess air or adding more air line to put in the stone(s) that you already have.
If you take a look at the lifting chart on the Vertex site, you'll see that when the diffusers are arranged together, they help each other with the lifting rates - i.e. 4 together have more lifting rate than 4 singles.
Also, if you take a look at all the different systems that are offered, only the smallest one has a single diffuser - all the others have at least 2 diffusers hooked together.
I run a single diffuser in the winter in 3'-4' of water, and run it at 5-6 psi. I want a large boil to create a large surface distrubance to keep the hole open. I just bleed off the excess air because the pump puts our a LOT more air than what I need for that single diffuser.
