I'm running a pair of Onix Ref1s, with a Ref100 center, off the 801. All three are 4-ohm speakers. There's a UFW-10 sub, but that has its own amp.
In the HT, driven by a 901, I have 7 various Rockets... 750s, 200 center, 300 sides, 250 rears, and a self-powered PC Ultra sub. Most of the Rockets are 6 ohms, but the 200 center is only 4. So because the 901 (like the 801) only supports one ohm setting, I have it set for 4.
I haven't had any problem with either overheating, and I will occasionally drive both sets quite loud. I think you'll be fine with pretty much any 4-ohm speaker.
As far as 'how capable'.... keeping in mind that the majority of the sound pressure comes from the bass, and that's on self-powered subs on both systems..... VERY capable, they'll both get incredibly loud.... to the point that I'm unwilling to be in the same room without my fingers in my ears.
The 801 is rated at 100wpc, and while I haven't measured myself and haven't read proof one way or the other, I'm nearly certain that you'll get every watt. Very nice receivers.