Wait, isn't your Denon 80 watts per channel? If so, I would think you have enough power for the Thiel's, at least for now- and, if you don't try playing too loudly.
If you need a decent low cost amp, you could try to find an old Adcom GFA-555, but they are a bit long in the tooth.
I would suggest finding a used McCormack DNA-1. or .5. The best thing is that you can send it to SMcAudio for upgrading down the road.
A Belles Aria integrated would be another suggestion.
Unfortunately, they cost a bit more than $300, but when you move up to a high fidelity speaker, you have to get higher fidelity equipment to power it.
FWIW, I think the Denon will be fine, for now. I've driven 4 ohm speakers without issue using a receiver pf lesser caliber years ago. It sounded pretty good, too.
Bob