How many meters are you from your speakers?
Do you listen loud or normal?
I like to have around 30db of headroom for maximum dynamics, but it's an ideal not entirely required.
That's around 105db
Your speakers have a 91 db sensitivity. You double power for every 3 db.
32 watts get you peak THX reference at 1 meter. For every meter back you double the wattage
64 watts at 2 meters, 128 watts at 3 meters, 256 watts at 4 meters.
If you are sitting within 10 of your speaker feet you can get THX peak reference with no issues from the 708 receiver. Beyond that external amplification might help. It might be a complete waste too. Since that is very loud.
What is the size of your room?