Where can you get a multiport MIDI-interface nowadays?
In my experience it is difficult to use more than 16 channels with any Amiga-sequencer since both Bars'nPipes and Music-X support multiple banks of 16 channels, but compatible hardware was never really big-scale and has only become more scarce.
Where should I look? Is there a DIY-solder-solution?
There used to be a professional 4x16ch interface from the same company that offered Musix-X itself. I think it was called the Pro-X or something similar. It also hat SMPTE-sync connectors.