I’ve had a bit more of a play with the sound-ported dulcimer, but my feeling is about the same. The sound going towards the listener is probably a bit more “open”, or “fuller”, but not a lot. I can’t really hear much of an effect unless I lean over the instrument, and if I did it again for myself, I’d put the port on the side facing me to get that extra bit of feedback. There does seem some enhancement of the upper treble fretboard, which I like, but at the expense of a slight loss in the lower fretboard bass. There’s no free lunch, unfortunately.
The lowest air resonance was raised from 227Hz (A +40c) to 250Hz (B +20c) with the port open, but the second air resonance (blowing across an upper bout hole) was unchanged. The note obtained by blowing across the sound port was 247Hz (B). I don’t know whether it’s just a coincidence that it is so close in frequency to the lower bout sound hole note or not.
Side height is 50mm.
Here’s a picture.
Richard T