Several Healey friends and I have been running the mild steel muffler sold by Healey Surgeons. Inan recommends it as the quietest one they've found. Still sounds good after 2 years.
I've had OEM, Moss mild steel (10 years ago), Pacesetter Monza, Moss TT and now the Healey Surgeons. The latter is the best. It doesn't have the nasty resonance at cruising speed the others all developed. Am running it with Kirk headers and 1.75" Monza tailpipe with Monza tips.
I've also had several different tailpipe diameters. Found by experimentation that going back to the stock 1.25" pipe does not run quieter than the larger sizes. FWIW was measuring the decibels with a phone app.