360 la headers & straight pipe

Flow masters use a baffle shape and location that can reflect waves which do effectively amplify, depending on the frequency. Some people love them, others not so much.
Mufflers or resonators can have a huge impact on volume and tone. Plenty of books out there explain how. Suffice to say, proper system design matters.
With headers and no Mufflers, it sounds like your engine simply lacks the performance attributes that make an engine sound gnarly - cam and compression.