I haven't built the Sontec, but I LOVE my DIY Calrec. It's one of those EQ's that make the source seem to sound better no matter which way you turn the knobs. I'd describe it as musical without being overly colorful, but it's not quite mastering grade transparent. It's an excellent EQ for creating a tone, rather than for adjusting a tone that is already defined. A perfect example is EQing a DI bass guitar. It shapes the bass in a similar way that the tone stack on a good amp would, which not all studio EQs are good at in my experience.
I hope that helps a little.
-Chris