Its impedance is rather flat at 6 ohm.
They are not very easy to drive IMO.
Its easy to see why when you look at the pdf. The speaker has 4 ohm impedance in the bass frequencies and rises to the 8 ohm impedance of the Festerex at 550Hz. I would call this a 4 ohm speaker and no mistake. Its not what I would call 6 ohms. The design suggests an amplifier with ’voltage source’ characteristics should be used, which means that most of the amps suggested so far won’t sound right on this speaker without the help of a set of ZEROs (
www.zeroimpedance.com).
The kind of amp needed to drive this will be a tube amp with lots of feedback or a solid state amp. However, you can assume from many of the posts here that most of the people on this thread prefer amps with little or no feedback.
If you wanted this speaker to work easily with a tube amp, the best solution would be to put the woofers in series and redesign the crossover (creating a 16 ohm loudspeaker). I expect the Festerex is considerably more efficient than the woofers; its likely that after such redesign the speaker would also be more efficient.