The HP Sprague-Rappaport (also later made by Philips) is supposedly better than any Littmann out there. If they brought it back and priced it reasonably it would probably still sell well.
I find that while I enjoy Littmanns (I have a III and will probably get a Master Cardiology soon), they tend to be louder as opposed to clearer. It's not volume but clarity that allows you to hear the most subtle sounds.
For example, I have a mitral murmur that comes with some regurgitation, a mid-to-late-systolic click, and an S3. My Cardio III will allow me to hear all of those pretty well. My Welch-Allyn Harvey DLX was actually the first stethoscope I used that allowed to me hear the click and made the S3 louder because of its bell.
With Littmanns, it's important to have the earpieces facing forward when you put them into your ears. Otherwise you won't hear anything. I do like that the Welch-Allyn Harvey DLX has separately adjustable earpieces in case you need to angle one forward or backward more. The Littmanns are set permanently at a certain angle coming from the manufacturer and aren't meant to be adjusted that way.
I also have an MDF Sprague-Rappaport. While it's better built than most of the knockoffs, it's still not as nice as the Welch-Allyn or Littmann stethoscopes. Double tubes are indeed nice, and I think they're still better sound-wise than the 2-in-1 design.
Overall I think a lot of it is preference. For us, buy one of each so you have options to use at different times. 😃