JonL,

Is the system going to be mainly used for music?  If so, then I can see justification in the 5Ts for the surrounds.  If mainly for movies, I think you'd be better off with bookshelves or, better yet - if you can place them to the sides of your primary viewing position - 5DBs.

If you stick with the towers, I don't think you are going to get any weird sonics going on with the 6B v the 5T even in all channel stereo.  I think they'll blend in nicely.

But if it is mainly movies you are going to watch with the system.  I think going with 5Ts might be overkill since the surround tracks don't get a great deal of sound information in comparison with the fronts.

And you are correct on the 10D/12D.  I think the 10D would work.  The 12D, though, gets a bit deeper than the 10 does.  It's kind of how much you like your bass.  Since you say you do, might want to opt for the 12 then.