I think thats the point I was making above. 100db/w is only a measure of how loud it goes for a particular level of input. Whether a speaker requiring a really complex crossover sounds any good driven by a low-power amp is a different question.
Not that the STA25 is low-power, it's 35-40w, indeed in my view it's one of the best EL34 PP amps you can buy (there are people out there trying to run Westminsters with 6w of 300b SE, solely on the sensitivity performance with no other consideration) but it might explain why you didn't quite get what you expected.
One of the biggest problems with the Westminster Royals is all the internal push connectors used for the wiring and crossovers. They make the sound dull and inconsistent and only get worse with time.
After removing them and soldering the connections, sixteen push connects can be removed from the signal path. Also bypassing the front panel treble settings with an internal jumper on the H.F. crossover board saves going thru those wonky connectors, improving treble clarity.
Doing this makes a HUGE difference in treble clarity openness and consistency. It’s a must do if you own them.
Also, the three point mounting system with one foot under the center of the front baffle leaves the front baffle flapping in the breeze. They need a four-point mounting system so the front baffle is “grounded”.
Another problem is cabinet ringing. The cabinet needs to be damped to keep all the panels from resonating to the low frequencies. There are some large empty (unused) spaces inside the cabinet that create the folded horn shape. These need to be filled with sand in order to dampen the cabinet resonances. And yes it turns a 300lb speaker into a 600lb speaker.
Bi-amping with an active crossover and connecting the amps directly to the drivers makes the biggest improvement of all. They take on an electrostatic type of openness and clarity.
Westminster Royals can be great speakers but you have to work at it. My late friend Harvey “Gizmo” Rosenberg used to compare it to working on hot rod cars. He had a point.