Speakers for triode tube amps

From what I’ve gathered, the impedance curve of the driver and the ability of the amp to manage the peaks and valleys will very much dictate how well the combo will sound.

Damping factor will play a role when it comes to how well the amp will control the driver, hence the need for a comparatively flat impedance curve.
High sensitivity is great, but it doesn’t tell the full story as to how well the speaker will react to the amp.
Tube amps have traditionally had low damping factors. Lots of modern speakers depend on high power amps with high damping factors to manage the impedance swings present in the speakers. The wrong speaker specs could provide an output that has low output in some areas and high output in others.

Cant wait for an opportunity to hear Fritz speakers in person. The crossovers he uses makes for an easy load for the amp to see, no capacitors and series design. The tweeter and woofer become part of one system, turning the two drivers into basically a single unit.

And then of course there is the all important room… As frequencies are shifted about from that, and pending how the impedance curve interacts with the amp, combined, components which have the capacity for wonderful reproduction on their own could end up providing a lack luster presentation.

You could also consider some kit speaker-Audio Note,Seas A26 and some of the Troels Gravesen designs suit lowish powered valve amps.

Fyne Audio F-series will be fine on 25 watt push pull Luxman 

Samuelg,

I hope some of the suggestions above have been helpful.  It is a bit hard to give focused advice, given that we don't know some basics, like a price range for instance, but, you are probably looking for a wide range of options.  For example, I like the suggestions of the Devore speakers, but, I have no idea if they are in your range as a choice. 

I know you mentioned that you have the Harbeth 40.1s, so you are not looking at Harbeths, but, I think that their other models sound substantially different from that speaker and the other models are MUCH easier to drive than the 40.1s (these are particularly demanding speakers that can sound dead or flabby with the wrong amplifier).  Perhaps the 30.2s  would be a candidate.

I again mention the Audio Note speakers.  It is interesting that you have ProAc and Harbeth speakers, because one of my favorite dealers has ProAc, Harbeth and Audio Note as his primary speaker lines.  He also has Nola, although for some reason, they are not that popular (the cheaper models sound good for the money to me).  This dealer, DejA Vu Audio (Northern Virginia) only sells tube amplification, so their speaker preferences are matched to this type of amplifier.  Most often, when someone comes in looking at all options (i.e., they did not come in looking at just one specific model), they end up with an Audio Note speaker.  

If you can stretch a bit in price, Charney Audio makes terrific speakers that are in the 99 db/watt and higher level of efficiency and they are fantastic as long as one is not interested in extremely deep bass. 

Jim WIney and Bill Johnson solved this in the 1970's.

Audio Research and Magnepan marketed and sold their products in tandem.  They are more accurate than any other combo.

There's the answer for you--no big mystery here.

Cheers!