When the round speaker gets too close to the top of the shell, a very strong resonance sets in causing frequencies ~1KHz and a bit lower to suddenly become a lot louder. It is possible that you might get a better balanced frequency response with a 1925 mounted much lower.
Here’s a quick and easy experiment to do. Enable drive hold, and fire it up. Now take the flat palm of your hand and hold it horizontally over the speaker. This simulates the top of the shell. Now slowly bring your hand down and towards the speaker. At some point, probably around 1/2 inch or so, you will hear an abrupt increase in high frequencies. I just tried this on my LokProgrammer with a 1931 connected. The emphasized frequencies sound almost like a steam hiss, perhaps accompanied by a loss of bass.
You might also try this with a 1925.
All the best.