Just went through all of this myself.
Transformers allow you to use low impedance mics (i.e. AT853) with high impedance inputs (i.e. line-in on a pcm-m1).
For now, I have: AT857 (same spec as AT853) > AT power module (can use samson instead) > Hosa Lo to Hi Z Transformers (can use AT transformers instead) > line-in on my m1.
I plan on adding a ps-2 (phantom supply) between the power module and transformer ASAP, so the mics can be used to their full potential.
If I don't use the transformers in the chain, I have to go mic-in on the m1 in order to get a good recording level.
(Mic-in resistance is only 4.7KOhm, where Line-In resistance is 47KOhm on the m1) As everone knows, going mic-in is not desirable.
Attenuators reduce the gain of the signal when it is too hot for the input (basically opposite role of the transformers). Like pfife stated, you would probably have to use them if you were running an AD-20.
Redbook: I haven't used the JB3, but I assume that you would have to use the transformers between the ps-2 and Jb3 if you want to record line-in. (Not sure what the line-is resistance is on the JB3)