It's not likely there is a muffler that will sound "deeper" - the bike is a small displacement four after all.
Quieter than stock will be hard to do. The predominant sound from a stock bike is the primary gears whining.
Is your exhaust totally stock, and is the muffler OK (not rusted out inside)? It should be rather quiet after all.
The Supertrapp mufflers aren't as quiet as stock (and get louder as the glaspack guts shred and blow out) but sound about the best of what I've heard if you don't desire GP bike style wailing. Their multi disc outlet lets you tune for more or less noise (and proportionally less or more backpressure to help with carb tuning).
Exhausts sold in the UK and many other European countries have to meet fairly rigid noise specs, unlike North America where anything goes - and the makers goals are apparently to be LOUDEST!! (all well and good until you get a noise ticket from our "servants and protectors").
Getting a Euro exhaust, if you can find one for an antique bike, should provide a pleasing legal purr. And that whine.
DSS has (had?) repro stock exhausts if you like the "wall of chrome" OEM look and find your old muffler is no longer muffling.
