I will add just a couple of cents into this discussion. I, personally, think it is in bad taste to start suggesting other brand of speakers in someone's forum, such as Rick's. There is a thing called PM that works great in this case.
The OP asked for a type of speaker that is for a type of amp that Rick, afaik, doesn't really aim to design for, and I agree that the vast majority of SET amps have horribly high output impedance yielding unfavorable measurements with most speakers (look at a previous comment for more on that). I have built a single ended (although not Triode) amp myself and owned a few others, but they are utterly colored. I think Rick favor's truth based on listening to two other speakers he designed and seeing how he designs.
A speaker mentioned (not Rick's) that might meet some of the OP's objectives (not sure about benign impedance, didn't look...and further I bet Rick could design something similar if he so wished) is the Fusion Tempest mentioned earlier. I think I heard the 15" version of these and they were great. More typically used for HT aficionados but if the impedance was benign enough could also be used with tube amps. Might still want to add a sub.