Hi Teflon,
It's me the guy who runs that Facebook group, my name is Jan Woestenburg, Johan De Greyze is an alias.
It contains about 180 backing tracks mostly from Shadows originals.
I have been using iZotope RX7, which is a pretty good program, but Spectra Layers Pro 8 has just some extra. In combination with Roland R-Mix (Cakewalk Sonar plugin) it is good for a lot of successful lead extractions.
Roland-R-mix can be compared to the free plugin Mash Tactic and is able to remove a window from the stereo image that is determined by frequency and position. So a lead with for instance frequencies of 250Hz - 5kHz in any position can be removed.
In Spectra one can disassemble the sound into layers. It has algorithms for vocals, piano, drums and bass. What is left is put in the "other" layer. It also can disassemble a tune (or layers) into components like Tonal, Transients and Noise. Noise may contain a part of reverbs, but there is also a reverbs reduction algorithm.
My way of working is to split a tune into stems, then export the "other" layer into my DAW and use R-Mix to remove as much lead as possible without losing the rest. Then reimport the result in Spectra and do a remix.
In general this creates a good result. There is also a possibility to do "fine tuning" by removing specific frequencies that are still in.
Spectra also enables remixing mono recordings. I did this for Driftin' that is only available in mono (studio version).
Have a listen:
https://app.box.com/s/d81nqg0h19yhpdrdflqx9wg0t9gxxwevJan