DC in the CIC-FIR filter result

Looks like you have overlooked dc gain in your system. Most likely your cic filter.

In fact cic filter or fir or mixer must take care of dc.

for cic dc gain = (R*M)^N (R = 64 in your case, M is comb stages, N = cascade stages).

If dc gain is only due to cic then if (R*M)^N is power of 2 you can discard lsbs to get unity dc gain and so avoid designing division.

For fir if sum of coeffs = 1 (normalised) then it is ok otherwise rescale them.