Randy,
You can find all the particulars on Paul's website.
https://sites.google.com/site/undefinition/core-2-way He includes graphs for frequency response as well as impedance. Per his description- it would be listed as an 8ohms nominal speaker. Based on the graph I would estimate a mid 80-s for efficiency. I am powering mine with a Marantz at 50wpc, and had them at a pretty loud level last night, and that was only half volume. Also drove them with an older Kenwood integrated amp for testing- which is rated at 40wpc- and had similar results. I would consider them pretty efficient and amp friendly- as they are driven easily with my receiver. Tube amps- I am not familiar with, so I cannot speak intelligently about how tubes would drive them. But I would think with at least 15-20 watss- that would be plenty to drive them.
Jeff