I'd advise against the Littman Master cardiology. I have tried both tunable diaphragms and conventional separate bells and diaphragms. The latter has superior acoustics.
Stethoscopes obey common laws of physics. Overall, most conventional stehoscopes will perform quite well. If you want the best acoustics, you should get one with the shortest length tubing that is well insulated (which is why the serious cardiology stethoscopes are all no longer than 27 inches). However, I have to also advise you that I know of a number of colleagues in Emergency Medicine who intentionally sacrifice acoustics to have longer tubing. In the ED, there are occasionally patients that you don't want to (or can't physically) get too close to the chest. Just FYI.