Kenwood named it 'sigma drive' for marketing and it was quite the 'thing' on their mid-upper range integrated and power amplifiers for a few years. It disappeared due to reports of amps blowing up due to incorrectly wired or dropped contacts. I have seen a few M2s which blew up for no reason and I blame incorrect sigma drive wiring. The M2 also had a commutating supply with an HV rail for high powered transients which didn't help stability either.
Back to Sigma Drive, it is a great concept and something I messed with several years ago too. The goal was to remove the speaker cable effects; resistance particularly, from the output equation as the speaker cable represents a significant proportion of the overall impedance particularly if the cable is long and thin. By essentially putting the speaker cable partially in the negative feedback loop, you can create a 'negative impedance' to cancel the cable's resistance/impedance.
Yamaha did a similar thing in the late 80s with AST (active servo technology) where they coined 'negative impedance drive' which took the characteristics of AST labelled speakers and used a matching plastic cartridge (fit into the back of AST amplifiers) which had a network inside to offset the resistance and compensate for frequency response anomalies in order to have a small speaker reproduce a flat response. Clever and effective.
Sigma Drive is, and was a valid concept for improving damping factor, just misunderstood.
