I know nothing of the shop mentioned above. I will say that in all of my air suspension experience one thing I have learned is never have a stereo shop doing suspension. This shop may be different. But without knowing their work personally, I can't say.
They did recommend a great company. If you owned one of a hundred different cars. Just not a Scion. Air Ride Technologies (ART) makes nothing for the xB. One guy on the SL forums has an xB that was bagged using ART components. He bought the car from the original owner, who adapted the ART parts to the xB. In my opinion, they did a very poor job of it as well.
I posted to you in the other thread about the lambo doors. There are a couple shops in SoCal I would recommend you talking to. Devious Customs, Go Ez Customs, and Chassis by Aaron Iha. They can all do excellent work. But they would be TOTAL OVERKILL for installing air on an xB. If you are willing to spend the bucks for good equipment, you can also talk to Autofashion down in San Diego. They sell Air Runner air suspension.
Lastly, if you are up for a drive and are willing to come up to the central valley (Visalia) let me know. I can work out some pricing for you to do an install.
Hope that helps.