Try using the lever on the top of the filter for flow control.
That said I think it's better to reduce volocity rather than flow if you can. You can do this by making the holes bigger on the spray bar or the outlet. This should reduce the force of the water (so fish can swim easier) but not reduce the flow. I think the more flow you can get the better always.
So the first filter I got in the mail got crushed by Fedex
Had Pet solutions overnight me another one and crossed my fingers that Fedex wouldn't break that one too!! It arrived all intact this time so I went about setting it up. Wow this is a powerful filter! This is actually replacing two filters that I had- an Eheim 2028 and a Magnum 350 on my 120 gallon tank. This is by far more powerful then both of those put together-even full of media and with an in line reactor. I am really happy with my choice.
I actually ended up not using the eheim spray bar, but I did use the inlet. I also set it up a bit different than before and it really made a notable difference. I ended putting the inlet and outlet on the side of the tank and using the included "jet output" or whatever you call it, setting it up about 3 inches below the surface and aiming it up to break up the surface film that tends to accumulate. The circulation has never been this good in that tank before. Good current to all the plant leaves etc...It actually was a bit too much for my liking though so what I did was take a Dremel tool to the outlet and open it up quite a bit. Doing this seems to have reduced the force to a degree that I think I can live with and the fish seem to be happier not fighting the current so much.
As far as noise the filter is a tad bit louder than the Eheim, but way more quiet than the magnum 350. One think I tried was replacing the impeller shaft with a ceramic one that I had from another filter. It has make a small difference in noise level and also seems to increase the flow slightly. Just spinning the impeller by hand you can feel how much smoother the ceramic shaft is compared to the original chromed metal one. Either way once the cabinet is closed I can't hear anything its so minor.