Dear TEd: I think you just slobbered a bibfull. "Low down" could have the predecessor of "the bottom line" but not talking about profits or tax returns.
I don't think "the low down" and "the bottom line" are quite synonymous. "The low down" is like inside information, such as Mr. Jones in shipping announced his resignation to the boss today, but I already knew he was quitting because he gave me the low down last week on his new job over at FedEx."
"The bottom line" is like the synopsis of a long-winded tale. Mr Jones' mother lives in Memphis and she's been ill, plus he's been dissatisfied here in Portland, and there was an opening over at FedEx, and well, the bottom line is he's quitting his job and moving to Memphis to work for FedEx."
There's a somewhat related phrase: "Keep it on the down-low" or shortened: "keep it on the DL." This basically means that you don't want people to know about something; to keep a secret. I not really sure how the two phrases relate though. I think this one is more recent.
I've never heard that expression before. It makes sense so that means it will probably spread. I only know of "keep it on the QT" which I always assumed stood for keep it quiet.