Disclaimer: I'm not into a wine-tasting mentality with compressors, EQs, etc. I don't
need an emulation of the legendary EQ from a legendary mixer that was used in some legendary studio to record a legendary piece of music; what I need is something that will cut and boost at frequencies of my choice in different ways
That said, you might find
this article interesting, as it describes how to use noise to evaluate the QuadCurve EQ (as well as other EQs). One of the QC's limitations is that it's not phase-linear, but SONAR has a phase-linear EQ so you're covered. For an interesting test, set up the Sonitus and LP-64 EQs to the same high-frequency boost settings, then switch between them on something like ride cymbals...you'll hear a difference.
For me, the main limitation of the QC is the number of bands. Four bands isn't really enough for mastering, although it's a toss-up whether you want to use the QC or the LP-64 for mastering anyway. I do like the Hybrid mode when there's program material with resonances that need to be but also needs gentle boosts; the "Pultec-like" Pure mode is great for general tone-shaping. So when I do use the QC for mastering, I usually feed into a bus so I can get two QCs in series and end up with 8 stages.
The other limitation is the spectrum analyzer. It's a welcome addition, but doesn't allow for averaging, hold, etc.
But as a general-purpose, highly versatile EQ, the QC is tough to beat. I think some people who have a strong preference for a particular EQ might not be aware of the effect of the different QC curves, so they choose an EQ that defaults to a curve they like. Its's really important to become intimately familiar with how the QC curves affect sound so you can use the right curve for the right job.