Hello, Ben!
Let me first thank you for your interest in our product, as well as the kind words offered up by Duke and Mariusz.
Despite what a lot of people think, I believe a horn speaker such as this benefits from a bit of power in order to be able to control the large voice coil inherent in most 15" woofers. While a lot of folks talk about using very lower power SET amplification, I don't feel that is the ideal match for this type of product.
So, I'm in the camp of your 40 watts of tube being a potentially good partner for this loudspeaker. The question I have is, what is the tonal character of the Leben, specifically, is it more on the rosy and euphonic or insightful and engaging side of things?
Of course, the ultimate answer as to whether or not this is the right product for you can only be answered via an audition. And, to that end, if I can assist in that, I would be more than happy to help in that.
Thank you again,
Joe