Power requirements
Dallas,
You picked a great receiver for your HT. Your speaker choices make a good entry level setup, but their quality and performance is not on par with your receiver. Your receiver has ample power to drive all of your speakers to distortion, so be careful, or you might fry a couple of the small satellites. Thus an external amp is not necessary. Only consider an external amp if you buy some inefficient 4 ohm front channel speakers.
The amp section of your yamaha is known for being transparent. In contrast, tube amps are know for coloring the sound and rolling off the highs, thus it would undermine the performance of your receiver.
Concerning speakers, it is recommended that all of the speakers tonally match, especially the fronts and center channels. Depending on your budget, I would recommend a floorstander or bookshelf system from Axiom Audio, or other speakers of equivalent performance to untap the potential of your receiver. For a decent 10" or 12" sub, check out SVS or HSU.
You are off to a good start with your new receiver purchase and you have come to the right place for more information.