340 muffler decision

E55D21, those are the welded UFs and they will be louder than the more oval st. st. Ultraflows. Those are packed with a bit less sound absorbing CRF (fiber) and were typically a bit higher in sound levels when we checked them. Mostly, like you said, during acceleration and that is why they have that more blapp blapp sound. Tailpipes help in some cases but amplify the blapp in others.
Tom