I decided to go with a Dominant A for now to match my unwound gut G and D. For the C extension, I decided to try a Kolstein VariCor. After using the Spirocore (Mittel), I was in for a rude surprise - a very obvious reduction in volume especially arco. One thing you have to say about Spirocores - they may be bright and they made not be easy to bow but they are loud as hell and have a strong fundamental. Low C on a Spirocore is like a pipe organ. So I'm back on the Spirocore low C for now. The question is, what other strings might produce a similar volume and strong fundamental and be good for pizz but would bow better? Surprisingly, I'm not really interested in a darker tone although a little bit darker/warmer wouldn't be bad. I'm beginning to realise that with the lowest string and especially with the extension notes, that a string which is too dark really does not cut through very well at those low pitches. I'm thinking Superflexible or perhaps Jazzer which according to at least one person is apparently (surprisingly) really good under the bow.