The block is looking for a user-generated list of tab-delimited FIR coefficients with a .fir or .csv file extension. It will accept up to 384 coefficients at a 48 kHz sample rate. AFMG's FIRmaker is but one program that can generate FIRs. See http://firmaker.afmg.eu/
It is admittedly not necessarily for a casual user. It's primarily intended for loudspeaker manufacturers who have the tools to generate the proper data.
The coefficients are integers that are mathematically integrated with a sample (called a tap) that produce the correct response for the filter. It actually is a very old technology. Was used in software data modems 30 years ago. With DSP's dedicated to processing the equations you can build devices like the 8800, it simply awes me.
This is just a WAG based on my engineering background but I don't think you can improve on the data provided by the manufacturer even if you had the software to generate the coefficients. The designers have access to manufacturing data on the parts, monte carlo analysis on the tolerances and measurements taken with task designed test equipment in known acoustic environments. It's not "tweakable".
If I am wrong I am very confident someone will point it out!