Different beasts.
The Radford STA15 series 3 has a sweeter midrange to the STA25 and is well suited to match with the Quad ELS 57 electrostatic loudspeaker.
The STA25 series 3 has a more robust sound especially in the LF region and is a better match for the Quad ELS 63 electrostatic loudspeaker imho.
A endless debate as to which is best, for my tastes in music, the STA25 wins hands down using conventional "Non Electrostatic" loudspeakers.
Edit: I do recall seeing / reading your published articles in HFW,
so sad to witness its demise and by those dreadful reasons caused by a minority.