What to say... B1 is a great design by Nelson Pass. Easy to build, absolutely transparent to the sound, compact, PCBs are available. I built several, using both PCBs from PasDIY and my own, made from Nelson Pass' gerbers, in various enclosures. Below are two builds: one with a relay selector (gerbers are available, although in hindsight, six inputs are way too many; three would be enough), the other in a slim 1U enclosure. Both use high quality Goldpoint level controls.