I still believe "Daxis" is Karr's homage to Richard Allen Davis, the killer of Polly Klaas. The "x" indicates death or elimination. Davis is a pedophile and a killer - two things that Karr seems to admire. He doesn't want to sign the e-mail as "Davis" because that would lead to many possible misunderstandings but "Daxis" (in my conjecture) means he is in kinship with Davis, the pedophile/killer. I think he sees Davis as his role model of sorts.
I have no idea what S.B.T.C. means although I've heard the theories. None of them ring true to me at this time.
He also called himself Damon in his "rock star" persona stage - then named his 2nd son Damon.
Early on in the case, a man (think his name was Wolf), was considered a possible suspect. He had a sweat shirt with the initials SBTC which stood for Sana Barbara Tennis Club. Not much more was said about his after a cursory look.