Dear Dick,
If I understand Gerry Romack's Helmholtz Effect theory, then the packing would not have changed the symmetry of the "acoustic chambers" of the mufflers. The symmetry with the equal pulsing of both exhausts creates at a certain rpm/frequency of pulses the reasonance that reinforces the other. That range of rpms that generates the reinforcing waves creates bigger waves which result in the notorious boom.
The Romack fix alters the acoustic chamber so that they are no longer symmetric and that, apparently, disrupts the re-inforcing sound waves and reduces the boom. The asymmetric mufflers have new resonance at non-complimentary rpms and therefore do not reinforce each other to produce the boom.
In Gerry's case he did pack the muffler with steel wool packing at the same time as he did the mod. He reports that his exhaust sound was very much reduced. Then he took it on a chassis dyno and unintentionally blew out the packing. The exhaust level did increase somewhat, but the Pantera boom remained greatly reduced. Also, if I recall correctly, that dyno test resulted in a measured *increase* in HP over an earlier test with the normal muffler configuration.
I intend to modify my GTS mufflers this year.
Warmest regards, Chuck Engles