Recorded multitrack stems remain scarce for modern separation and generative audio models, while large synthesized alternatives often depend on closed commercial instruments. SPDMX is a public-domain multitrack stem corpus from PDMX: scores are cleaned, rendered with open soundfonts and selective MIDI-DDSP, then loudness- and velocity-aware mixed into mono stems and mixes at 44.1 kHz—sharing song identity with PDMX so symbolic metadata stays usable alongside the audio. Song-hours count mix duration once per song.