I was wondering if you had tried the Yamaha P-25, Lowboy. Mine is a yellow P-25F. I think there is also a D, but it appears to be the same. The P-25F has the holes flat on the bottom (I assume that’s what you mean, as opposed to along the side of the bottom). The vent slots are situated in such a way that they’d be easy to block or tape, with eight toward the blow hole and three toward the spit valve end. I just tried sealing them with scotch tape and couldn’t notice its making any difference. But I also tried blocking the three slots with the fingers of my right hand and working the eight slots with the palm of my left hand and it definitely produces effects. It’s also easy to produce effects with the spit valve button if you’re holding the melodica bottom in your left hand. This is a nice melodica; I’ll be surprised if you don’t like it.
I’m one who commented that the Yamahas were harder to bend notes on. But not so hard once I got the feel for bending.
The Suzuki A-34C also has its vent holes flat in the center of the bottom. It’s much lighter than the Pro-37, and it’s just a bit longer than the M-32C.