OK, I am still needing to learn much.

I can understand the chart at:
https://www.decware.com/newsite/buyingguide.html

So if your speaker efficiency is 84dB or up to 99dB you can see which Decware amps work the best.

But, I feel I'm in a Monty Python sketch asking "African or European," regarding an unladen swallow.

Is that chart for an 8ohm impedance speaker, or 4 ohm?

And, I know it has an effect, but will 4 ohm speakers be OK to use?  It seems they should be, I keep thinking maybe even better than 8 ohm speakers, but that's something I don't know.

Wow, thank you Steve!

I really haven't checked, but for the Rachael, is there such a switch or jack?

In the owner's manual it states,"
The SE34I.3 is designed to have it’s maximum power into 8 ohm speakers. That does not rule out all 4 or 6 ohm speakers, or 12, and 16 ohm speakers. It simply means that statistically it will work best with around 8 ohms. Since the only thing that happens when you get a lot higher or lower than 8 ohms is the power output goes down, having efficient speakers may make this completely a non-issue. You can not hurt the amplifier by hooking up speakers that are more or less than 8 ohms. This includes planar magnetic ear-speakers (headphones) which are too hard to drive with conventional headphone amplifiers but may be as high as 30 or even 70 ohms."

I don't understand what it means by"it's maximum power into 8 ohm speakers" and "the power output goes down."