Looking for recommendations regarding low-power high thread-count server for small ISP

I believe I'm looking for suggestions for low-power high thread-count servers (a pair) for a small ISP's DNS servers.

Low-power because one has to sit on a smaller amp circuit, high thread-count because that's what i currently believe is more suited for the application (powerdns or unbound, linux).

But if your experience says my assumptions are not so good, i'd love to hear what i should be looking for instead. This is entirely caching/forwarding DNS servers, not authoritative. For less than 10k customers.

Thank you for your time!