Yeah John isner uses an o3 white, but i am friends with a former athletic trainer that worked with the tennis team who said Isner has some deal with Prince, i think he gets racquets but not money. I'm sure they are customized a bit, otherwise i dont know how he would keep the ball in play (he's 6'9 and very powerful). I wouldnt really call him a serve/volleyer though, maybe he volleys more than the other players on the team but not enough to define his game, its more powerful serve and big forehand to put it away.
The usual number two player, Travis Helgeson, uses a K6.1 95, not sure of string pattern. Luis Flores, the #3 (which is really unfair to say, because Isner, Flores, and Helgeson really could rotate at #1), uses an 03 tour ( i think the 100 sq in. size). Matic Omerzel and Ricardo Gonzales both use technifibre (not sure of models), Jamie Hunt and Nate Schnugg (the guys who won junior US Open doubles last year, lucky r-e-cruits [dont know why that word is not allowed]) are using Wilson Ntour 2's and either Nblade/Npro open(couldnt get a good look), respectively. Josh Varela, the sometimes #6 with really cool double backhands, rather than a backhand and a forehand, uses what seems to be a stock pure drive. I'm not sure about the others on the team, haven't seen the rest play.