Thank you, I can't wait! I've bought the non meta ones for a reasonable priceMy budget has a limit unfortunately.
The non meta ones are fine imo and I doubt I could hear such minor differences. They are still real 4 ohm speakers though phase angles weren't bad.
It seems if you are okay with the weak NR1506 amps you will be fine with the C50, my concern about heat is only if you suddenly want to listen much louder, say >6 dB louder than you usually do, for a prolong period. In any case, I would still say it is worth spending $20 to protect the very nice and quite pricey AVR you now have.
Midrange AVRs are just not designed for use with 4 ohm nominal speakers such those relatively big R11 towers, though they can be used safely under certain/many use cases (such as yours).