The Logans are rated at 4 ohms.They might be conservitally be rated at 4ohm, but they do dip to 1ohm in the highs, with "may" see a dulling/recessing of the highs with many tube amps, unless like the Roger Modjeski RM200 Ram Tube amps, that have 1, 2, 4, 8ohm transformer taps, then enough watts become an issue.
Because the lower you go in "tap ohmage" the "lower the watts become" also, so a 100w at 8ohm tube amp may become only 25w at 2ohm.
Cheers George