Vincenzo

No loudspeaker is a purely resistive load and it isn't even an attribute I'd say was related to quality. Most of the better speakers don't have all that flat an impedance curve. In fact, most of the better speakers depend on electrical damping to perform their best. So the amp is a big part of the deal. Things like output tube, transformer and configuration come into play as does feedback.

That's why I've begun to see the levels as being too grey to really talk about tube-friendly speakers. One amp is fine with just about any speaker and the next is incredibly picky. If your amp is picky, then you might want to go with the Studio, Theater or Pro speakers. All the sevens are fairly benign. But I'm not sure I'd want to narrow my choices like that. If an amp is that picky, I'm not sure I'd be happy with the amp in the first place. Too many other excellent choices that aren't so finnicky for me to live with an amp that is.

Bottom line is I doubt your 300B is going to have a problem driving any high-efficency speaker, but only you will know for sure. It isn't a matter of watts, it's a matter of reactive loads and current sources. So probably, if your amp sounds good to you on other speakers, it isn't so delicate that it needs a purely resistive impedance to sound good.