

Rtings.com provides frequency response charts when they test speakers, let me see if I can find one that goes low enough for you.
As for the bonus question, absolutely love it. I love when people come up with cool solutions to life’s problems, and this lets me hear my own heartbeat? Hell yeah!
Here’s an easy way to check a bunch of them quicky, the Denon home was the best I found in terms of the very low end. I’m sure someone else can do better, I’m not that much of an audiophile and know very little about speakers.
Probably worth noting that they stretch out the low end of the chart so they tend to go lower than it seems (I assume it’s a logarithmic scale). You might be able to go to a store and ask to test them yourself.
Second edit: if headphones are at all a possibility (they might be better for patients who don’t want to hear their own heartbeat) then can I recommend Skullcandy crushers? They’re completely ridiculous as regular headphones, they basically just have a metal plate they vibrate for the bass, but it goes all the way down to 20Hz, and you can crank it waaaaay up with a slider on the side. (I use these daily for music because I’m a bad audiophile who wants to have fun sometimes, I have a proper wired setup if I want the audiophile experience.)
I’m afraid I can’t answer your technical questions, I’m really not that knowledgeable about this. All I know is you ideally want the frequency curve to be flat, I don’t think it matters much where it sits relative to that line.
Honestly, that Danon dropping off at the low end is pretty typical though it’s one of the better ones. You’d really just have to test it I’m afraid, it might be totally fine to chop off the bottom for some things but maybe it’s necessary for certain heart conditions, I wouldn’t know. If it were an option I’d say the best bet is to always stick to the analogue, but I’m absolutely with you on hating traditional stethoscopes, they’re so painful…
You probably can just leave the crushers on your neck with the volume maxed out, but I’m really not sure if that’ll work. In all honesty the speaker might be worse due to the way the acoustics in the room can change what you hear, it’s really hard to say.