Ohm load for SET Amps

What sort of problems have you heard in those speakers and what sort of budget do you have?

Halving the impedance doubles the current (amps) required. Tube amps are current "challenged" compared to SS amps.Therefore the higher the speaker impedance the easier it is for a tube amp to provide enough current.

The 4 ohm tap allows the amp to make the current needed. The problem is that limitations in the output transformer often mean that the transformer runs a bit hotter when running the 4 ohm tap, and this extra heat comes from the power of the output tube itself. IOW, the amp will make slightly less power due to its being less efficient. But it will make the current needed to drive 4 ohms.

IMO/IME there really is no good reason in high end audio for 4 ohm speakers regardless of your amp. They allow you 3 db more output if you have a solid state amp, but **all** amps make more distortion into 4 ohms as opposed to 8 or 16 ohms, and that distortion is audible as brightness and additional harshness, neither of which is desirable especially in high end audio.

If you really want to do your amplifier investment dollar justice, a higher impedance speaker will do that better all other things being equal. In the case of an SET, I recommend a higher efficiency speaker too; an 845 doesn't make a lot of power as amps in general go, so about 98db 1 watt/1 meter would be my minimum recommendation if you really want to hear what the amp can do.

A 16 ohm tap means that ALL of the OPT's winding (on the output side) is in use. An 8 ohm tap means that 1/2 of the OPT's winding is in use. A 4 ohm tap means that 1/4 of the OPT's winding is in use. This directly affects damping (control) of the speaker's motion.

Actually this isn't true. The 4 ohm tap has half the windings of the 16 ohm tap. The impedance goes up by the square of the number of turns- so the 4 ohm connection is actually the center tap if there is a 16 ohm tap.

Output transformers tend to become less efficient into lower impedance loads. This means that it will absorb a slight amount of power, but the other issue that is often more important is that bandwidth can suffer- by as much as an octave off the bottom from 4 ohms to 8 ohms!
The 845 isn't easy to get bandwidth out of anyway due to output transformer issues.
It really is to your advantage to seek out a 16 ohm speaker- the amplifier will not only have wider bandwidth, but slightly lower distortion too.