We have been auditioning a lot of types 10 and its cousins..
To name a few:
Sylvania 10 (tungsten)
Hytron VT25 (tungsen)
Western Electric VT25A (oxyde)
Sylvania VT25 (oxyde)
theForest Audion 510 (oxyde)
RCA 801A (tungsten)
And also some more boutique:
Elrog TM801A (tungsten)
Elrog 801A Mob (tungsten)
These all share the same form of the plate, box-like, some curved at the edges, all with the cathode in between, evenly spaced. Plate materials differ however, cabonised metal, graphite and even molybdenum.
These all have their specific sonic characters and it is all about preference and fit in your system..
The one that stands out for me is an oldie, a sort of predecessor of the type 10? Almost the same specifications but way differently built: The Telefunken RV210. Tungsten cathode (same power), big tube base and different socket type using 4mm bananas. Dagerous as well..
But these have a nickel plate which is oval shaped around the cathode. And physically a lot bigger. These have something special, musicality in them that is not present in all the others.
So.. most probably it is lack of linearity or maybe distortion.. don't care! Love how these fit in my system!
All were tested at Vp 385V, Vc ~30V and Ip ~18mA using a choke input power supply with 5R4G (RCA) rectifier, MuseCoils transformers and chokes..