On my system, there might have been a touch more detail with the SI op amps, but I found the upper midrange a bit harsh sounding and distracting, particularly on less-than-stellar recordings. But I can't say that this was due just to the SI Op Amps. I've made a lot of changes to my system over the past several months, and it's only been the last few days that the system has really started to gel.
I just replaced the pre-amp to amp interconnects today (Madgrigal CZ-Gel I to DH Labs Revelation). I'll have to give it some time before I decide whether this is improvement, but I fully expect it will be based on how much the Revelation between my DAC and Preamp improved the sound. With this last update, I now have a full DH Labs loom which I'm quite impressed with.
Based on the sound I had earlier this week (before this latest cable swap), I'm now feeling that this amp with the Sparkos is pretty damn good. Excellent dynamics, beautiful vocals, and deeper-tighter bass than my Krell FPB-300 was providing. Imaging is also the best I've heard in my room. Again, I have made a lot of other changes, so I can't say that it is all attributable to the Nord, but a system is only as good as it's weakest link.
I think if you have other components that result in a laid back midrange, the SI op amps might be the better choice. I could imagine a system with a tube pre-amp and/or DAC might work well with the SIs, but I definitely prefer the Sparkos in my system.
Whatever you decide on, give it lots of time before you come to any conclusions. It was literally months before the system really started to sound its best. My speakers are quite efficient though, and my room is fairly small, so perhaps a system that pushed the amp a bit harder would get you there faster.