What is the appropriate horsepower (wattage) for a new pair of PSB T2's?

They are rated at 89db.  I'd call that medium efficiency.  You do need to keep in mind that doubling the wattage only gains you 3db.  Perceived volume increases with every 10db increase.  
The NAD is rated at 80 watts.  That's ok but not a lot.  However, to hear a difference you will need to at least double that.
As said by some of the folks above, wattage isn't everything. Large power supplies, dynamic headroom and such make a difference too.

There was a poll over on diyaudio some time ago where people measured their maximum power requirements using their own amps and speakers. 71% needed 25W or less with only 16% needing over 100W and that is the maximum power they needed not their normal listening requirements. If you don't mind reading lots of pages about people questioning the maths the thread's here.
If you do go for a lower power amp (and there are good reasons to) then you just need to make sure the designer hasn't skimped on the power supply. I rate mine to deliver a sine wave at full power into the lowest rated impedance so they're easily up to the job... others don't always want to pay for that much copper.

Thank you for all the responses. I guess my point was/is I moved up from a fairly decent bookshelf speaker to the T2's at $3500 for the pair and I was expecting more fidelity than I am experiencing. Don't get me wrong but I like the T2's, I just would like them to project a consistent sonic experience at all volumes and I think the NAD C356 has a lot to do with that. I realize I misspoke talking wattage only, I guess I was looking for recommendations for upgrading the Integrated amp for something that was more supportive of the T2's. 

NAD C375 would be a better option. I have Synchrony One B, 88db.The C356 will not give you the best of the T2's.