Here is an update. I finally replaced the rear muffler with the Dynomax 17670. The muffler went in the same spot as the Spintech muffler and it was a tight fit. I was overall 6-inches longer than the old muffler (26 inches total length).

The car is now MUCH more enjoyable to drive and it about 50% less loud than with the old muffler. No more droning.

As a reminder the exhaust is a Single 3" to a Camaro style muffler that goes in the space of the original Volvo muffler. I am very happy that I did't go with a Dual 2.5" exhaust for the following reasons

1. Dual 2.5" vs Single 3" have very close flow
2. There is not very much space under the 240
3. Weight gain...