Hi there, Mike!
I know it’s been a good number of years since you made this post, but I am on the same page as you and am currently searching for a way to acquire this wet sound on a concertina. I am in love with the concertina as an instrument, but seem to appreciate the sound of a small, wet accordion much more. Maybe this means I should get over my aesthetic desire and focus on the music, but if I could keep the joy of playing a concertina while getting this wet sound, I would be overjoyed. Did you ever find a satisfactory option in your search? If so, I’d love to hear about it!
many thanks,
Stephen