The 6DB is circa 1978 or '79. It was sort of like the original 6000d, except it didn't have TR disc I believe. So it was sort of the little sister "poor man's" version of the 6000d, as I recall. Guys back then did real well with them! But of course, they are dinasours compared to today's deeper seeking and slower motion disc's. Those old motion machines had be swung REAL fast to get depth.
As to which is better, I'm not a B.H. fan in general (nor have I used the tracker IV), but even today's low end machines probably work better than yester-year's power-house machines. If you are in an area prolific with targets, then I suppose it doesn't matter. But if you're in a competitive area, where you are going behind others, then 30 yr. old 6db technology, or a BH, may not get you a lot behind them. It all just depends on where you intend to hunt.
As for "gold", I should caution you that the 6db favored high conductors, not low conductors, just based on the frequencies and such. So for example, it would get a dime (high conductor) deeper than a nickel (low conductor), even though the nickel is the physically larger of the two coins. That was true of most of the early fast-swing motion machines.