I spoke to Mike at Heartthrob this morning. Part #101 is an IMCO stock type muffler. He said it is very quiet. The HVS is their "free flow" muffler and the HPM is their performance turbo muffler. The HPM is the quieter of the two. I copied the info below from their 40 page online catalog.
Exhaust Flow Tests for 3” exhaust
CFM Backpressure
Heartthrob HVS #54144 248.5 0.25
Flowmaster #943042 233 0.50
Heartthrob HPM turbo # 1142 o/c 222 0.25
Heartthrob HPM turbo # 1142 c/o 218 0.25
IMCO #914 turbo 182 0.666
IMCO muffler #101 (stock type) 140 0.50
Link to their catalog:
http://www.heartthrobexhaustinc.com/2004cat.pdf