Edge of breakup GOLD
I ended up buying a number of other overdrive pedals (some very expensive), but this Spark Booster is the one that does exactly what I need. I was looking for something to put a little hair on clean guitar patches on my GT-100 to fit in a mix, and that I could just leave on all the time. I use the mode switch set on 'mid', with gain 08:45, level 11:00, bass 09:15, and treble at noon. The guitar is connected to a TC Electronic HyperGravity compressor, then to the Spark, the GT-100, and a pair of PE60s. The Spark gives a beautiful tube sound with the exact right amount of breakup when I hit the strings hard... perfect! Tried it with 4 guitars (humbuckers, split coils, and mini-humbuckers), and the transparent breakup sounds just right with all guitars on every clean patch.
Decade ago had oh so many tube guitar amps, but nowadays I only play and record at home. Nothing else I've tried comes close to the Spark for that full clean tube-like (6L6) breakup. TC Electronic makes the best pedals that I can afford.
In closing the Spark Booster is magic, and that's as good as it gets. And thanks again for the help, Philip!