First, you have to know that I am, BASICALLY, a Classic Series fan. By I am referring to the tried-and-true Classic SL series that was discontinued.
With the 'original' Classic SL series, ALL models operated at 6.59 kHz. The Classic I SL had a fixed 8" coil, and the Classic II SL had an interchangeable 8" coil HOWEVER it did not share the same Blue Max series of coils as the rest of them.
The Classic III SL featured a Frequency Shift as well as a Normal/Black Sand toggle that simply expanded or enhanced the lower discriminate adjustment range.
The Classic ID was based on the simple 2-knob control design of the Classic II, but it also introduced the "flash technology" Target ID of the Classic series. It also came with the 8" Blue Max coil standard.
The Classic IDX was essentially a Classic III SL with the Classic ID Target ID circuitry added. It was supplied with a Blue Max 950 coil.
The IDX Pro was mainly a name change (to the "Pro" series about 6 or 7 years ago, but there was a very slight enhancement of the upper-end Sensitivity. Most (including me) could seldom tell any difference. The main advantage of the IDX Pro was a change in the decals to a non-glare decal below the TID display. I sure appreciated the elimination of sun glare! The IDX Pro retained the same 950 Blue max coil and it, like the Classic IDX, Classic ID and Classic III SL, could use all of the "Blue Max" series of coils, to include 4", 3x6 DD, 6