The trombone has more "presence" in the upper register (at least up to high C). The bass trombone is much bigger than the tenor trombone. So this puts the poor 2nd Trombone at a disadvantage. For maximum volume you can double the 2nd trombone part so it balances the 1st and bass when they are at "blastissimo".
Of course if you are only using 3 players it might be a good idea to ask the 1st and Bass to dial it back just a little...

Good points/observations. In one of the ensembles I play in, we have four trombones. So, we typically double 2nd.
