First, remember to not confuse the model transition. There was the 6000 Di Pro SL which used the larger control housing and was a little heavier. It was replaced by a newer, re-engineered model called the 6000 Pro XL that used the smaller-size 'Slim-Line' housing and had the slide-in 8-AA battery tray. That very same model had a simple name change about eighteen years ago to simply the XL Pro. All the same internals and performance, just an outside decal change to eliminate any numeral reference and be just the XL Pro. Inside the battery door, however, is does have '6000' prior to XL Pro.
Either of these models, the 6000 Pro XL or the XL Pro, are some of my favorite White's models and I have bought them ... and then sold them ... in the past 6 years for anywhere from $275 to $395, and some came with 2 coils. I see two on the big auction site asking $500 or more and I wouldn't pay that much as you can find them more affordable than that. So don't be fooled by how well they perform unless you can check them out; don't make a mistake on the production model name; and don't over-pay unless you are very certain it is a unit that you really want and will use in order to get your money's worth out of it.
Opinions are free,
Monte