Custom instruments for playback

I write a lot for percussion ensemble. I like to write for an instrument called Songbells, a metallophone that sounds an octave below the Glockenspiel. For this instrument in MS, I create a glockenspiel part, rename and change the octave transposition. For generating parts, this is fine. For playback, not so good.

Below a certain note, I don't get any playback for songbells. Is there a way to create a Musescore custom instrument that actually plays back the lower note range? I would prefer with the basic glockenspiel sound palate, rather than vibraphone, since I also write for vibraphone.

I've attached a sample if you'd care to inspect.
Thank you!