Yes, just have it flicker once in a while.
I assume the H300 has rockbox? Set the AGC to "safety clip", and set the gain on the H300 to about +5.
Then adjust the headphone knob to get a nice signal on the iRiver. Once you have that headphone knob nicely adjusted, put a piece of tape over it and leave it alone.
From then on, get in the habit of leaving the headphone knob alone, setting the iRiver gain to +5, and then use the UA-5 individual gains to make the H300 happy, which should be at that point where the light flickers occasionally.
If the H300 sees a little hot spot, it may automagically turn down the gain a notch. If the gain drops below zero you are running too hot into the H300 and you may start to get serious distortion.