I have the Exmark aerator that I bought last spring. It is an awesome machine. Yes it is expensive, but it is a production machine. I ha aerated from 6000 Sq. Ft. to baseball & football fields with it. I tried the Classen aerator and this thing is far superior in every aspect. I also have a Turfco TurnAer 6 that I am going to sell. As far as the pressure gauge I just start at 300 psi and raise or lower the pressure to get the core depth I want. My son used it on his golf course on the tee boxes and said it did great. I highly recommend this aerator to anyone doing an appreciable amount of aerations.
Also, studies by Michigan State has shown that aeration has no effect on pre-emergent effectiveness.
Mike's Total Lawn Care
I love my country, but mistrust my government, and the government doesn't create jobs, it just takes my money and gives it to other people or groups it thinks it can bribe.