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

The main factors in deciding what power you need are the size of your room, type of music and your preferred listening level. Those speakers are fairly sensitive (90dB) so unless you have a particularly large room then you probably don't need anything greater than 50W and can probably get away with a fair bit less.
If you have a digital multimeter and a digital source (DAC etc) then there's an experiment you can do to work out how much power you need at your preferred listening levels. If you're interested I can find a link to the instructions.

Regardless of wattage needs, there are definitely some speakers that benefit from amps with higher current output and/or higher damping factor.  I've heard this with two amps demo'd on the same speakers.

Technically speaking, horsepower affects top speed volume while torque has more of an effect on acceleration. This is why even a lot of solid state horsepower seems less powerful than a smaller amount of tube torque. The greater tube torque accelerates the voice coil zero to 60 much faster. Remember all the world's top speed tests where horsepower rules are run by professionals on a test track. In the real world where hardly anyone has a test pressing and no one has a mastering studio its mostly listening at normal volumes in normal rooms. Where torque is more important than horsepower.

So again I ask, why do you think horsepower matters?

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.