Is it possible to do math on some of the subband's. E.g. pull out a few subband's and sum them up?
Yes, you can do math in subbands. You may have already seen the modules in the 'Frequency Domain/Subband' and 'Frequency Domain/Math' folders. Many other blocks will handle complex data. Examples are Adders and Block Extract, which allows you to extract a range of samples from a block of data. Also, you can always convert subband signals into real and imaginary signals and operate on those signals using modules that only support real data.