Hi Ian,
accidentally I have noticed that the last version of Bassmidi that runs on non-SSE CPU is the last official one namely 2.4.14.1 The other versions I have (2.4.14.8 and 2.4.14.14) both give an exception 0xC000001D right after BASS_MIDI_FontInit/BASS_MIDI_StreamSetFonts calls so I cannot even call BASS_MIDI_StreamCreate.
I tested this on PCem 17 that properly emulates earlier CPUs without SSE. Bassmidi versions 2.4.14.1 and earlier work perfectly with PCem emulation (P1 120, P1 MMX 166, P2 233).
I don't know if this is intentional or not but I cannot see this new requirement documented anywhere.