NA 2 row is in the 150-160 lintner range these days. 6 row has gone up to 180 Lintner from what I have read. Those are both so high that there is little difference, so like Denny says, you don't need it for the high enzymatic power anymore.Both Rahr and Briess also have 6 row, but I only used it once in an American Pre-pro lager and really could not tell any difference. I also can't recall if it was an American barley or not. I was told that the 6 row has higher diastatic enzyme action, so it can help convert more adjuncts and has a bit mor of some type of saccharides, but I don't recall the specifics at the moment. So, give any brand a try on a known recipe and see if you notice the difference from 2 row. I would like to hear your thoughts.
FWIW, these days 2 row barley has almost identical diastatic power as 6 row. It is no longer necessary to use 6 row just for added enzymes.