My wife and I tandem. Our all-up weight is about 350 lbs. We run 25mm tires like Conti 4000s or Schwalbe Durano at 120 psi very satisfactorily. We run 28mm tires like Michelin PRO4 Endurance (sold as 25mm) or Vittoria Rubino Pro Tech at 110 lbs. We don't ride much on gravel and find no need for wider tires. We run 36H Velocity Deep-V rims with no problems.

Edit: I forgot to mention that when we tour, our all up weight is 400 lbs. and we tour on the Pro Tech 28mm tires at 115 lbs. We like the PRO4 Endurance tires so much, though, that next tour we might be on those.