Actually, the last of the '6000' was the 6000 Pro XL. When White's realigned their models and came out with the E-series they also changed the IDX, Quantum XT and 6000 Pro XL. The IDX became the IDX Pro, the Quantum XT became the QXT Pro, and they dropped the numeric '6000' reference and swapped the rest of the title to make the XL Pro. Other than the name and the decal/paint to reflect that renaming, there was no change at all. Exact same unit.
This two-named model differs a good deal from the earlier versions of the 6000 series. The 6000's progressed thru the Series 2 and Series 3 and then the Di Pro seiers. Even the Di Pro went through several circuitry changes (revisions) from it's release in April of 1985 until near the end when White's settled on the 'CB' version (Composite Board).
The 6000 Pro XL/XL Pro uses the smaller control housing and the drop-in 8-AA battery tray of the newer models, and the same excellent analog display meter they have used. But the 6000 Pro XL was a new detector. It wasn't a repackaged 6000 Di Pro SL 90 because that circuitry wouldn't fit in the smaller housing. The engineers at White's set out to match or better the performance of the former 6000 models and achieved it with the 6000 Pro XL. It only carried over about 15-17 parts from the earlier model.
I love my XL Pro and I am sure than when you learn yours well you will, too.
Monte