Unwanted offset attenuation in FIR filters

The HSMC ADDA card has AC(transformer)-coupled in- and outputs, so it's clear that a possible DC bias does show at the output.

The ADC and DAC data representation is unsigned, to process the data without conversion, it's O.K to set the FIR data format to unsigned, as you did. If simulation/SignalTap waveform formats are also set to unsigned, the signal should show centered. The unsigned data representation is only applicable for a low-pass, it wouldn't work for a high-pass or bandpass filter.

Personally I prefer a signed representation for digital signal processing. In this case, the ADC data has to be changed from unsigned (offset binary) to signed (two's complement) by inverting the MSB.

The discussed 6 dB attenuation may be caused by the signal scaling of the ADDA board, but I'm not sure about.