Ron,

As has been said, the 590  has  several EQ choices in the radio, as well 
as  the ability in the ARCP-590 software to tweak it even more. 
Everyone's voice is different, and what works for you might not work for me.

If more money and additional complexity are no problem, you  could add a 
microphone switch box.   This would let you use your desired mic, and 
let you switch to the  MFJ mic with the 4 pf keys  when you need to push 
a button.    Not sure I would go this route, but it's an option.

On a sort of semi-related note,  I see the MFJ-1251 is a microphone 
conversion box with internal jumpers to  match different microphones to 
different radios.

I don't know how large the enclosure is, but it could possibly be the 
basis for a homebrew pf key box.  I know it uses small internal 
horseshoe   jumpers, and someone would need to lay their hands on a 
couple of  jumpers with wires  to break out the 2 pins needed to go to 
the buttons/resistors that would be the new pf keys.

73, Steve KW3A