know some people (me included) have had issues withe the level of the synth compared to the guitar.
The mod on Joness excellent site is probably the best way to solve this.
However if modding not an option. Yo could try this.
Yesterday when i was playing. I connected the GR-300 like this.
From the mix/ synth outpour into a FX10 Bi-FET Preamp and into a Boss LS-2 return A
Then from the guitar output into return B
Just put a cable in the input of the LS-2 which noting connected to it.
By adjusting the A and B level on the LS-2 and boosting A (mix/synth) with the FX10 Bi-FET Preamp I was able to get a good balance.
Then i just used the balance control on the G505 to chose from guitar or synth.
I'm not reinventing the wheel here, but this worked good for me!