Ohm load for SET Amps

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.

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. 

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. 

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.

It should be said, though, that for a given speaker impedance the damping factor, and hence "control" of the speaker’s motion, will be greatest from the 4 ohm tap, and lowest from the 16 ohm tap. Although the damping factor will usually be fairly similar for a 4 ohm load connected to the 4 ohm tap compared to an 8 ohm load connected to the 8 ohm tap, etc.

I’ve always heard/read/searched that 8-ohm or higher loads are ideal for tube amps. Speakers are "tube friendly". I’ve never really understood why that is the case.

To add to the other responses, pairing a SET amp with a low impedance speaker is especially likely to be problematical if the impedance of the speaker varies a lot as a function of frequency. That is because tonal balance will be affected by the interaction of the relatively high output impedance of most SETs (and many other tube amps) and that varying speaker impedance.

A higher impedance speaker having an impedance that varies in a similar proportion will exhibit that effect to a lesser degree, everything else being equal, since the output impedance of the amp will be a smaller fraction of the speaker’s impedance at all frequencies.

Regards,
-- Al