The one thing that makes tubes fail, aside from vibration, is high plate voltage, ( id not know if this is how it translates to English)
I have a PL 300i and my partner has 2x 400 in mono. I really like them but they are very dependent on the speaker efficiency.
With the 300 I have Tannoy Kensington and both of them are really good. I auditioned the Tannoy and Focal Sopra 2 at Upscale. When I was there my favorite music, Spanish Flamenco and guitar, sounded magical on the Tannoy and the 300, no need of sub where the Focal needed subs. But at my home in the US, definitely needed the subs, but they still probably being broken in.
At the courtesy of a peer that is using my home since December the speakers will have a few more hundred hours of playing and open up as the ones in Upscale.
With the 400s I tested a few speakers I own just to see how good the 400 are. The speakers were Tannoy Canterbury (auditioned in my house for 3 months) , Wilson Audio Sasha, and my 3 favorite Avantgrade Duo Primo XD, Zignali Client 3.18 EVO and Sonus Faber Aida.
The 400s are very good, however when you put great speakers behind them you start to see some not as good elements on them. I do not have a $10-15K tube amp to compare it yet, a Pathos Heritage is coming, but when I compare them to the Gryphon Diablo 300 (a $13K SS amp) there is no comparison on how they sound.
In the same db range, the Diablo sounds bigger and relaxed much more dynamic and refined and these qualities are consistent across a wider db range but the Diablo is a solid state and powerful so it is not all that fair. The Diablo surpasses the 400s the ability to make the Zignali Client 3.18, Sonus Faber Aida and Avantgrade Duo Primo XD sound as close as they can.
The caveat here is that I usually listen at low-mid volume (4-12, 12 being with the dial at the top middle in the PLs) which it is in my experience that PLs need a little more to get going lets say 10-1 or 2.
For the $, the PLs sound incredible specially the 300i and the 300s which I made the mistake not to get with a good pre, if I were to give my partner her PLs again I would have given her the 300s.
Would I buy them again, yep specially for a smaller room and be careful with what speakers you hang from them.
When I compared to what I internally saw to others, including the Raven in Kevin's video, the PLs are build with care and detail; the others at this price range looked like amps my son was building at 12. Some of them even dangerous given the voltages we are talking on a tube amp.
I am getting delivered a smaller set of Avantgrade for the 300 lets see how they sound.