The O2 was nice sounding, even out of the 323A and the T1W, but I actually liked the Lambdas more than the O2 running off those amps. I think a lot of the comments that people make about the O2 sounding dark fit how they sounded out of the 323A / T1W. The level of detail was still greater than the Lambdas but the O2 just seemed a bit flat in comparison. I'd describe the O2 as "good" on those amps.
With the KGSS they really do come alive. They go from good to excellent, and definitely beat any Lambda I've heard for listening enjoyment [to my ears].
When I purchased my O2mk1 I didn't own anything better than the 323A, but I went ahead and bought them because the seller gave me a really good deal and they were in near mint condition. I just couldn't pass them up. They didn't wow me at the time on my 323A, and it was pretty obvious that it was due to the amp's lack of power. Until I purchased the KGSS, they stayed in their flight case tucked away safely while my 404LE got all the attention.