I don't know your budget, but it seems that Tekton Lores ($1K) or Mini-Lores ($650) would be a good match. The Lores' sensitivity is 98dB with bass down into the 30's. That should maximize your available power. The Mini-Lores' sensitivity is a claimed 95 dB, which would also get plenty loud on 20 wpc.
An 8" or 10" ported woofer should be pretty representative of the load your receiver would have driven back in the day.
An 8" or 10" ported woofer should be pretty representative of the load your receiver would have driven back in the day.