Impedance match Fishman guitar pre-amp to IRIG2HD
The common approach is to use the matching T attenuator circuit. The impedance is not critical, as its a line system instead of a microphone. The simple resistor network I would recommend for this:







simulate this circuit &ndash; Schematic created using CircuitLab

This gives about 16db (15.5 or so) of attenuation, to get back to nominal guitar level.  You could use a passive DI w/pad, and a xlr to 1/4 inch, but this is much less bulky.