Seems that there is a little issue with vgm logger.
The only player capable of playing vgm right is GameEmuPlayer (MAME based afaik). But when played on emulators, even RetroArch (using VGM_PLAY V3.42 rom builder), the PSG is pitched down...
Cannot test it on hardware right now though, maybe it's fine...
Compared to deflemask vgms - they sounds more or less fine everywhere.
--- upd
I think culprit is BBC Micro PSG clock used (4000000) instead of native Genesis/MD 3546893 (PAL) or 3579545 (NTSC)
ym2612 clock also may be better to change to Genesis/MD 7600489 (PAL) or 7670453 (NTSC, prefer that) instead of 8000000. Then VGM merging will work much better.