But I'd have to get another amp or run 4 channels on the receiver if I wanted 7.1. The UPA-5 is $50 less than the XPA-3. Unless you were referring to another brand of amp.
What Sholling is trying to tell you is that you don't need to use all or none of the receiver's amplifier channels. You can elect to have a 3 channel amplifier power your fronts and the receiver to handle the surround channels, which it would have plenty of power to do after relieving the fronts off of receiver duty. Surround rarely requires significant amounts of power, and when it does it's short lived. Even at the most tremendous peaks of surround power required the Pioneer wouldn't run out of steam.
LOGICALLY you should just get the receiver, listen to it with your speakers. If you hear stress from lack of power then consider getting additional amplification. Don't confuse mechanical stress with amplifier stress. If there is mechanical stress that means your speakers are limiting performance and all the amplification in the world won't change that. There is no reason to spend money on something if you don't even know that you'll need it. As mentioned before your Polk speakers are fairly easy to drive and that Pioneer receiver is more than enough to drive them. They make receivers for this reason. They don't make receivers so you have to buy additional amplification. Since Polk has been in the mass market they've pretty much made sure that their speakers are designed to work well with receivers.
If you get the amplifier now you may be wasting $800, rack space, and electricity and pushing your significant other's patience if you have one. That extra $800 or so will be better directed at a subwoofer, or a pair of subwoofers if you so desire. I honestly don't believe you'll need additional amplification with that receiver.
I've had my current receiver for 6yrs and I never checked for firmware updates for it.
Well that's not what we would consider a modern receiver, there's probably no user interface that allows for firmware updates like today's receivers.