The mechanical filters in the 75A-4 and R-390A cause a similar degrading of audio quality. That's why the R-390 (non-A) sounds so much better than the "A" model.
the 3.1 kc mech filter in the 75A3 was intolerable. the cheap ceramics are significantly better audio partly because of the shallow skirts. The thing that's kind of cool is weak signals stay in the 6 dB ~flat response curve; stronger signals (that you'd want to enjoy with good audio) push their way farther down the skirts a bit for a wider response. There's some double peaking going on across the passband but I don't care--I'm not trying to work DX. I don't see spending the high dollar amounts for the A line mechanical filters I see advertised in ER. Maybe if I were going to do some serious signal digging.