The best answer to the low volume with the earphone jack problem is to bypass the resistor that lowers the signal level at the jack. Easily done on most scanners, like I did on my GRE/RS PRO106, Uniden BC235, etc.
I haven't opened up my PSR800 yet to look, but considering the "high tech" design of the radio, I'm wondering if they might use an attenuation method other than a simple resistor. The fact that the radio reports the jack is being used makes me wonder. Anyone disected their radio yet??