OK... this is interesting...
I have (2) pair of the early single fret S45.
I have a pair each of the later double fret S45 and S55.
The reason I chose the S55 cab was because it was a 3-way unit (same as early S45) rather than the 2-way unit of the revised S45.
With all of the cabs sitting next to each other, I find it remakable that there is a volume delta of 7 liters. Granted the S55 is a bigger speaker, but, wow.
If I decide to proceed, I would do one at a time and of course, compare!