A properly designed muffler which is not tuned is also a very good choice -- the model of JMB I like is that type can - run on a very short header - the powerband is seamless from idle to full power.
Once you tune to a boost - there is a point --somewhre in the band which is the transfer from below resonant to resonant.
some "pipes" are extremely good at masking that point - providing th prop rpm is matched and th carb is set correctly -
The worst problem - adding large props which are used to reduce noise -
these really screw up any tuned setup and contrary to what some believe. reduce power band available.
I have used and do use all three types exhaust setups - depending on the application.
tuned pipe
tuned can
muffler only can.
of these the muffled only can is easiest to install and setup - and depending on engine /prop - far better than the so called factory in cowl devices.