OK first, please read the FAQ. Srsly. Second, true bypass is mostly hype. Buffers are not inherently bad, and in fact many, many guitarists and bassists think a good buffer is the key to great tone! Think about this: the whole point of an "active" bass is that it contains a buffer!
A bad buffer is bad not because it is a buffer, but because it was cheaply built and poorly designed.
Some Boss pedals are great on bass, so anyone who tries to tell you they all suck is showing childish ignorance.
As a broad rule of thumb, the guitar pedals that tend to have inadequate low end are dirts (OD, distortion, fuzz) and compressors. There are many, many exceptions!!! It's just a general guideline to consider.
Most other types of effects do not inherently lose any lows--though of course you have to go on a case-by-case basis, really.