First SRA1-8 amps were all classic tube units of varying complexity made in the 60's.

SRA-3S was the first deviation from this with a transistor front end and AC coupled tube output stage. Preamp and phono amp on plug in cards similar to the old ISA units used in PC's.

SRA-10S/12S were the first SS amps with a push pull front end and AC coupled outputstage. Run fully in Class A they get stinking hot and feature a nice regulated PSU for the low voltage circuit and a stacked PSU for the +640VDC B+. Similar plug in system as on the SRA-3S but now everything except the basic PSU is on plug in cards and the build quality is far better.

SRD-X was the first portable Stax amp and features an automotive chip amp driving a pair of transformers. Think Darkstar but with much better specs done in 1979....

SRM-1 Mk1 was pretty much the same circuit as the SRA-12S but turned on its head. Similar AC coupled output stage. I recently refurbished one of these and it is a nice little amp but the SRA-12S is better over all.

SRM-1 Mk2 is where things get tricky since there are so many variations of it. The first ones were some weird Mk1/Mk2 hybrid though fully DC coupled but later units are much more similar to the current crop of Stax amps like the 313. Now the amps are fully balanced from input to output though most only have RCA inputs.

SRA-14S is pretty much the SRM-1 Mk2 circuit but turned on its head. Better parts and some tweaks but most of the price difference is spent on the preamp capability.

SRM-3 is just a SRM-Xh with a full size power supply. It even uses the same circuit boards.

SRM-600LTD is the same basic circuit as the T1/006t but modified to take the ECC99 tube.