Assuming you already know that the "acidity" of flavor is totally subjective then I'd ask John at work a couple more questions. Well water or city water? How often he waters (is he more "dry farming" than most)? And what other fertilizers/soil amendments he uses through the season? Does he have any idea what his soil pH might be?

Since clay normally retains water much longer than other soils he maybe able to reduce his watering substantially over what would have to be used with other soils. That would have the effect of intensifying flavors - whatever they maybe.

Just a guess.

Dave

Try Break O'Day or some of the older varieties. Carolyn calls them assertive.

I thought this was an interesting thread about acid tomatoes:
http://forums.gardenweb.com/forums/load/tomato/msg1220161811535.html

Ya, I'm disappointed this year. My tomatoes aren't as acidic as a normal home grown tomatoes. They've grown in number and in size more than any other year, but the taste is blah. I added composted manure but not calcium. i do have some foliar calcium spray I can put on the leaves.

This thread is ten years old. Need to look else ware for newer information.