The Infinity RSIIb had slightly better EMIM magnets, and better woofers.
The list price was only slightly higher than the RSIIa
The 'b' IS a slightly better speaker than the 'a' version.
The one with midrange cones instead of EMIMs was very inferior.
As I wrote, the main feature is the rectangular EMIM drivers on the upper panel,
Three of them in a vertical array for RSIIa and RSIIb
(with two EMITs next to the, and one EMIT on the rear)
if you have three 3" round poly (clear plastic) cone midrange, then you do NOT have the 'a' or 'b'
OPS, if you fool around with the wiring to the EMIMS, be very careful to not stress the pins the wires connect to on the EMIMS, they will break off from vibration really easily, So best just do not mess with them if they are working well.
And replacement 'inner' parts were recently available, but they can disappear fast. So assume if you damage an EMIM, you are SOL.