Unwanted offset attenuation in FIR filters

I don't know why the signal doesn't show centered. Tracing the signal through the design by simulator or SignalTap should clarify.

I must admit, that I'm not using the FIR compiler for filter design yet, so I'm not aware of it's specific operation or possible issues.

Only a lowpass will transmit the input signals DC bias. Assuming the input signal is biased to midscale, it's surely the case for the said ADDA board. A lowpass keeps this property, a bandpass or highpass will have zero output bias, unless you apply an artificial offset. (I don't assume that the FIR compiler does for unsigned designs).

When changing the filter design to signed (my preferred representation for DSP), you have to change the AD data from offset binary to two's complement by inverting the MSB, and vis-versa for the DA output.