It appears that there is little difference between 8030A, B and C in terms of frequency response. However, they differ in:
a) Type of amplifier: A and B are using traditional AB amps, while C uses Class D, which drops idle power from 10 W to 3 W. While the diecast chassis would aid cooling, I would still expect the older models to run noticeably warm.
b) Input volume control range: 80 dB for A and B, a mere 12 dB for 8030C (the latter will be more prone to hiss if the output isn't super quiet)
c) Self-noise spec: <=10 dB(A) vs. <= 5 dB(A), although this may be mostly a paper thing (I don't think they could even measure that far down in the olden days)
d) Auto-Standby function (ISS): Only B and C have this, dropping standby power to 0.5 W.
The high trim appears to be identical (and not too useful) in all of them.
The model you are most likely to see on the used market would be the 8030A, being the oldest with a fairly long production run (2005-2013, vs. 2013-2017 for the 8030B). In the beginning it was still competing with the KH120A's predecessor, the K+H O110.
The K+H / Neumann models have pretty much improved with every generation: KH120A gained a larger enclosure (so presumably better levels down low) and a smoother frequency response, KH120 II is basically the equivalent of the older KH120D since it has a digital input, alongside substantially more powerful Class D power amps and a DSP that enabled deeper bass response. Idle power consumption has pretty much remained constant or even increased slightly (O110: 11 W measured, KH120A: 20 W spec, KH120 II: 15 W), although the KH120 II has finally gained an auto-standby function that drops it to 0.3 W.
Comparing Genelec vs. Neumann, the latter will play louder and have less idle noise, the former are more eco-friendly (these days an entire PC plus monitor might only consume around 30 W at idle, so doubling that for speakers may not be everyone's cup of tea).
