This is getting interesting. I think the previous owner had iit set up for a stock mic and for cutting through a pile up, which it does well with the setting he was using
I know that newer radios can access the balanced modulator directly thru a connector in the back. This will eliminate audio filtering in the mic preamp stage. Dunno if your radio can do it; but it is worth a shot.
I have an 890AT (actually two of them) and love it. I use the stock hand mic and also the MD-100A8X desk mic. Both mics sound good from the audio reports I have received. I have even used the MD-100A8X with my Flex 1500 and again, good reports. Yaesu radios, in my experience, tend to sound best with their microphones.
Other brands, are usually a bit flat without some tweaking. The MD-100A8X can be found (used) for around $100 and I consider it a bargin.
trucker