Re: clean boost pedals, what is good?
The boosters based on Orman's JFET mini-booster are really nice -- they are pretty true to the base tone, and may add a very slight touch of very musical grit. These would include the RGW Electronics Bad Bob Booster, the Burriss Boostier, the Luxury Drive, The Fulltone Fat Boost, the Pedalworx McBoost, and perhaps the Keeley Katana and modern channel of the TMB. Of these, I own the Bad Bob and have tested the Fat Boost over a couple of days. The Bad Bob adds more sparkle and operates with less noise than the Fat Boost, so I returned the Fat Boost and kept the Bad Bob.
The Catalinbread SCP and SPP boosts are based on Orman's MOSFET booster, and the ZVEX SHO and Super Duper are similar.
A number of other boosters are based on the old E-H LPB circuit, including the Nick Greer Sonic Boom and the Sweet booster. These add a lot of thickness to the tone and are darker than the JFET or MOSFET boosts.
I'm not sure what design is used in the SD Pickup Booster, but I thought it was kind of flat and dull sounding compared to the Bad Bob or a good LPB clone. I liked the resonance switch, but it lacked presence overall to my ears.
And then there are all of the treble boosters out there, but they are far from transparent.