In case it's not clear, the flow rate measured with no portafilter in the group (often referred to as 'water debit') is affected by the gicleur (jet) size in this machine. I think it's a 0.6mm ruby by default, although you can get larger ones (0.7mm and 0,8mm). That jet helps to moderate the pressure ramp up of the initial flow at the puck, not really the flow during the brew, exactly as Jim said earlier.
If you are for some reason trying to get a slower brew without tightening the grind, you may want to experiment with dialing down the pressure on your rotary pump. Easy to do, but in my opinion it would be best to first leave it at the conventional 9 bar and experiment with different baskets and different coffee doses to get the taste you're after.
If you want to change the pressure profile so as to get a longer/slower pre-infusion, I think you can do some of that by programming the pre-infusion times on the FB80. ( I may be wrong about that - am just going by what I read online, for example: La Marzocco FB/80 preinfusion?
For real on-the-fly pressure profile control you could resort to the suggested hack of adding an adjustable needle valve after the pump. It wouldn't harm the pump, but I suspect it might be difficult to shoehorn into a machine like this. It may be easier to trade in the FB80 for a machine that is designed to do pressure profiling.
You can find data on La Marzocco flow restrictors (gicleurs) and unrestricted (no portafilter) flow rates in this tech bulletin: http://www.lamarzoccousa.com/wp-content ... Guide.docx
* edit addition - Looking closer, I see that the OP has an EE model, which apparently doesn't have the programmable preinfusion feature.