I have those Grovers. The ones I got are sold through LiteGlow and mounted on a different manifold that what's pictured on the Grover site.
Shippy is right, the volume of air is the key, not so much the pressure. The more air you can run across the diaphram the better. I'm currently using 1/4" line and they sound pretty good. I was emailing with the guy at Grover on the best way to tune them and he informed me I should be using 1/2" line. So I'm shopping for that now.
If you want to hear the general tone of the horns watch Shipbldr's Paragon video. I'm hitting them in the first minute or two of the video.