I tried a couple of inexpensive passive preamps or attenuators and did not like the result. As I have a Dayton OmniMic2 system, I measured what happened. When gain was reduced, low frequencies were cut relative to high frequencies. This was clearly audible, as it was a matter of decibels, and the sound became less accurate.
I suspect that what I saw was interaction between the output impedances of the source and attenuator (passive preamp), the input impedance of the amp, and the balanced interconnect cables; still, I am not an electronics professional, so that is a guess. IMO, it is particularly important to try a passive before you buy, and if possible, measure the result so you can interpret what you hear. What sounds like "clarity" at first may prove in the long run to be reduction of warmth and bass.
Despite many positive reviews of passive units, I've never seen anyone else measure the results to ascertain what their side-effects might be.