This is one of the best documentation for a DIY speaker I have read. No head scratching, everything explained, excelent pictures.
Only thing that may be an option, is to do it active 3-way. Leaving out all passive x-over parts may be an advantage.
This could be done without or very little extra cost, if you replace the mini DSP by the DSPB-K DSP Kernel Board and DSPB-KE Kernel DSP Expansion 2-In 3-Out.
Which will mean a little more thinkering how to make the whole thing look neat and use the less comfortable programing in SIGMA Studio.
A nice alternative to the amp you use is the AIYIMA A07 or 07max. It is around 50$ a piece.