I've been using the 740 every Thursday, just because it's already there and set up. I set the DSP 4 to 3-band eq. Although this setting is not adjustable like the manual says, I haven't experienced any volume problems.
I think maybe if the DSP is set to compressor or limiter or some such setting, it is kicking in and affecting the mic volume.
While it is certainly not in the league with X1, or PSR8000 or Digitech external units, it sounds o.k. to me. And I'm very critical.
I think the key is in the e.q. The system there has full graphic e.q. plus exciter and good effects processors. It would make a Casio sound good.
Don