foo_3001 wrote:but I´m interested in what is the technical detail that allows the transparency / even bass response they have?
Well, quite a few people consider the Timmy to have a transparent sound. This pedal is similar in that the bass is cut in the same way - just different amount of cut and at a different frequency. The tone circuit is also similar to a Timmy - simple low pass filter. The quad of silicon diodes is also similar to a Timmy - they will give you a greater range before you notice their effect on clipping than something like just a single silicon pair in a Tubescreamer will.
The quad of diodes isn't unique to the Timmy. You'll find them in the Barber LTD and the old Marshall Bluesbreaker, just to give two fairly familiar examples.
The tone circuit isn't unique to the Timmy either - it's most well-known use in a similar configuration is likely to be considered the ProCo Rat.
There's a few ways that it can seem like you've lost the transparency, especially with the example of something like a Tubescreamer. Its fixed bass cut removes frequencies at around 700 hz and below, and there's no way to adjust the bass cut like with this pedal. It has a fixed treble cut that removes at roughly the same frequency. The tone circuit can then either cut more treble or try to offset what has been removed by the fixed cut that comes before it. So removal of the Tubescreamer-type tone circuit replaced with just a simple adjustable single pass filter will open things up quite a bit.
The Blue Note has a very small gain range, so it's not going to get very distorted or compressed sounding. You can think of it as having a portion of a Tubescreamer's sweep through the drive knob - about 60% of it, as a really rough non-technical estimation.
It's the simplicity of the filters that allow for what you hear: input / bass cut / gain and overdrive stage / treble cut / output. There's nothing else to really color things - the simplicity is good for users who like this.