Thanks greenstreet
While I was waiting for your reply I shot Bob at ESV a email to ask.
This was his response.
"Hi Mike,
Thanks for your kind words about our seawater system. We don't recommend altering the amounts of the Components A and B because they contain more than just calcium and alkalinity ions so you would be changing the concentrations of other important elements. If you just wanted to raise calcium then calcium chloride would work and sodium bicarbonate should work fine for a bump in alkalinity. Once you alter these parameters we can't garantee there will be no precipitate. It would be safer to use our salt as is for your water change and then tweak the aquarium chemistry with whatever method you use for calcium and alkalinity addition.
Hope this helps,
Bob Stark
ESV Aquarium Products Inc."