@mikewerner Here are a couple of things to keep in mind when looking at speakers for your amps.
SETs tend to be very high distortion amps, partially owing to no negative feedback, but also due their single-ended nature and the design requirements of the output transformer.
So to hear what they do best the first thing to know is that any zero feedback SET will have about 20-25% usable power. Above that level, distortion will cause the amp to sound ’dynamic’ due to how that distortion interacts with the ear. So you need a speaker that’s pretty efficient. In most rooms this means something at or above 100dB. It should also be at least 8 Ohms. 16 Ohms is preferred if you have a 16 Ohm tap on the output transformer.
The second thing to understand is the issues with reproducing bass frequencies. This particular problem is well-known to the designers of output transformers for SETs! Its one reason why a lot of modern SETs have very large output transformers for the amount of power they make.
Technically the problem is inductance, which is the property that allows a transformer to do its job. The power tube usually draws its operating current through the winding of the transformer. This imposes a DC magnetic field upon the transformer, causing it to saturate (distort). To reduce this problem, most SET transformers have a saw cut in the core (which is often described as ’gapped’) that breaks the DC magnetic circuit.
This works, but at a price. The inductance I mentioned is reduced below a certain frequency and continues to reduce as frequency goes down.
The tube is looking for a certain load to do its job best. With a 300b that’s about 3500 Ohms; to do that the transformer has a certain amount of inductance; the wire it uses to do that might have a DC resistance of only 200 Ohms. So now you can see what is happening; as inductance goes down, the impedance the tube is driving falls off significantly.
So when that happens the tube makes less power into the speaker, considerably more distortion and runs hotter (not a good way to treat an expensive DHT such as a 300b!) since some of the power it makes is dissipated directly in the tube rather than the load because of the reduced impedance.
On this account the simple and easy thing to do is to limit the bass going into the amp. It will immediately sound more transparent and coherent.
So you might want to consider using subwoofers to handle things up to about 80Hz or so and then prevent anything below that frequency getting into the amp. The difference, the improvement, won’t be subtle.
BTW the kind of horn speakers you need to really show your amp off often do not have much response below 50-60Hz anyway. Some dedicated subwoofer amps, like the one I use, have crossovers built in so with such amps its a matter of plug and play.