Your FIR filter length of 400 at 48 kHz is way too short for the amount of LF magnitude and phase filtering you are trying to achieve. Filtering at lower frequencies requires longer filters, as does aggressive filtering – high Q and high slope regions in the magnitude and phase of the filter. Generally speaking a 400 tap filter (at 48 kHz) will struggle below approximately 500 Hz, and that can be seen in the “FIR Filter Frequency Response” plot on the Export tab. Powersoft amps can take at least 2048 length filters now and the modules might be similar. I’d recommend switching to just IIR filters at the point at which the FIR can’t go any lower.
Maybe take a look at the Audio FIR Filtering paper
It has examples of differing filter lengths.
One other suggestion. On the Import tab, using “Normalise magnitude to max” will result in much of the filtering being boosts. Maybe turn this off and use the manual “Magnitude off (dB)” field to place the smoothed response roughly either side of 0 dB. In this way the filtering will be a mix of boosts and cuts. Although I’ve not seen it yet, it’s possible some processors will run into headroom problems in the FIR filtering. And gain can always be made up later in a separate gain stage in the processor.