As for the '6000' you and others have mentioned, it is hard to know which version of the '6000' series you had or refer to.
There was the original '6000' and it was followed by the 6000 Series 2'. That was then followed by the 6000 Series 3 which was my favorite of all the blue-box 6000 series units.
In mid-April of '85 they introduced the 6000 Di Pro with auto-trac, but it had several glitches and was easily out-performed by the Series 3. After they brought out the 6000 Di Pro it became the top-end model and the Series 3 was 'replaced' or discontinued. Many avid White's fans preferred the manual GB design and it was soon restored to the White's line-up, but renamed the 5900 Di Pro.
When they went away from the blue control housings to the black configurations they were still going through a few changes in circuitry design to work the glitches out of the 6000 Di Pro SL, and the final version with all the remedies used the 'CB' circuit board . The 5900 Di Pro also had a few variations, but the final production units incorporated the 'CB' circuitry evolvement, and I had a lot of very impressive success with the 5900 Di Pro SL 'CB' version, mainly using a 6½" Concentric coil for trashier or brushier environments [size=small](usually the Blue Max 600)[/size], and for more open, sparser-target locations I liked the Royal 800 or Pro-Scan 8" coils more than the 950 offerings.
Just a few short years ago I had to let my choice specimen go only because age, poor health and mobility issues have me mainly relying on lighter-weight and better-balanced detectors, most with a slow-sweep circuitry design. The 5900 Di Pro is in the moderate to faster sweep class, and the only detector in my arsenal that fits that group in my XLT, but it uses the lighter, smaller, 'slim-line' control housing with the slide-in 8-AA battery tray.
Put in the time to learn the 5900 Di Pro SL well and it will provide you with excellent in-the-field performance.
Monte