Guidance for amp/source/dsp

Hi, I want to get into DIY speaker building as I am very interested in the engineering aspect of speakers. However before a start a build I need to make some changes to the rest of the system and I would like some guidance on this. What I lack in my system today is a Spotify Connect streamer and a sub out and the ability to high pass the mains. Today my system contains of a Schiit Modi 3+ DAC, NAD C372 and Zaph Waveguide TMM speakers I built 10 years ago. I want to rebuild them to bookshelfs, or start over with new drivers, and eventually add a subwoofer with DSP.

I want to keep the cost down since I am a student so used market or even some DIY could be options. Im thinking of adding a Raspberry PI streamer with optical out, and then a miniDSP 2x4 HD as a DSP and preamp with a remote, and then use the power amp section of the NAD to the mains. This would allow for sub integration and the NAD would provide plenty of power. I am not sure this would work as intended tho so I would like to hear your opinions on this. Selling my amp and DAC and start over could of course also be an options. If you have any thoughts on this please let me know.