Hi there,
If the Quad has a Pre-out, then you can use that to connect a power amp. BTW your Int amps power section may continue to be active ( the NADs used to have the ability that the pre and power could be disconnected as the bridging was done outside..not sure of the Quad)
If that is ok and your speaker has the ability to be biwired then you may get a good upgrade by Bi amping your speakers ie running the HF posts of your speaker via the int amp and using a separate speaker cable ( duplicate of your regular) from the power to the LF posts of your speaker...