Why to low output??? Tip speed, transient response, detail, definition, etc. However the rest or you system needs to be up to it. Otherwise your wasting your money.
Great step up devices are available, and I prefer the SS version simply for the reduced noise. Use the tubes elsewhere. (just my opinion)
The reason its possible is the windings required to provide the gain are increased on the coils. This adds mass, and the force to overcome the mass becomes greater. You cannot accelerate a truck as easily as you can a car. The mass is pysical phenomenon, f= Mass X Acceleration. It can be done but the available force is deterimined by the record groove, and the mass loading on the record. Hence (in theory the low output MC (with lower mass) is capble of better performance).
Note: not all low output MC's are low mass, so choose your poison wisely. It can be worth it, but it is very expensive. I prefer a medium output MC and admit a stong liking for the latest Grado's above the Sonata.
Even the Sonata is a deal.
Also be aware that achieve the (potential benefits) of an excellent MC will require continous tweaking of at the very least VTA. I get very tired of all the futzing. So decide what inportant and makes you choices.
The mag reviewers get paid to tweak the snot out of the cartridge and report whats available. At the end of most days I just want to make on simple adjustment at best and let her rip. (probably a large portion of my fondness for the Grado's. Very good and not futzy.
good luck on your decision,