If I add a power amp to my NAD receiver....

Before adding a power amp, I would replace the ugly pins that come standard with NAD bridging the preamp out/power in with good quality jumpers. This will improve the unit substantially and you may find no need for a new power amp.

Depending on your model, you can do "both." You can add an extermal power amp while still using the internal amp for a bi-amping configuration, assuming you have bi-wire loudspeakers.

This was a nice feature of the C series receivers and power amps. These receivers had a "matching" power amp so you could combine them and know that gain would be the same for both. The pre-outs of the receiver could feed the power amps.

In fact, I think these receivers even had an adjustable gain control so you could level match a different power amp (instead of using NAD's matched set).