About 40 watts a speaker should be nice.
If you run Series you will see even less watts and about 10 ohms nominal at your amp.
Most amps are stable to about 16 ohms, but I nominally don't drive an amp with anything greater than 8 ohms or less than 2 ohms.
I have no experience on what will happen with 10 ohms on an amp optimized for 4 ohms. You should be ok but LACK volume (SPL) at 10 ohms series wiring.
