I'm having trouble wrapping my head around the concept of a "transparent" overdrive? What exactly does that mean? If a tone is being overdriven to any extent, does this not mean, by definition, that it is no longer transparent?
No worries...it's just me. Sorry to digress...as you were.
I hate using that word but I do use it from time to time. It's a quick and easy reference for me to use when I want to describe an OD that doesn't color my naked signal that much if at all. Just adds some drive or grit into the signal. Like @NKBassman said above, pedals like DG, certain preamps like a rusty box or VT that instill their own sound and flavor to the signal. That is not transparent to me. To me, something like the Pure Sky or depending on how you set it the MBD1 can be that way. But then I don't really want a pedal to be THAT transparent. Then what's the reason to use it?! I like different colors and shades and flavors to color my sound. Heck your amp and especially your cab in most cases will color your bass's tone the most.
I really want to try a TC Spark as well.