Kalali & George, I looked at the specifications provided in the service manual for the GFA-555 II, which can be found at HiFiEngine.com. While as Kalali said it is usual for sensitivity to be defined as the input voltage required to drive an amp to its maximum power capability, that is not always the case. And my perception has been that it was especially common for it not to be the case a few decades ago, such as when the 555 was designed.
In this case the 130 mv input corresponds to an output of 1 watt, as stated earlier in the quote George provided and as also stated in the service manual I referred to. But George, note that it says 1 watt, not 1 volt. 1 watt into 8 ohms corresponds to about 2.83 volts, as I’m sure you realize.
Also, the service manual states that an input of 1.75 volts is required for the amp to provide its rated maximum output of 200 watts into 8 ohms. Which can be expected to also mean that an input of 1.75 volts would produce the amp’s rated output of 325 watts into 4 ohms
These numbers correspond to a gain of 27 db, which is about average for a power amp.
Also, I note that the manual for the Magnepan MMG recommends amplification having 8 ohm power ratings of 40 to 150 watts, less than the GFA-555 II is capable of.
So a digital source having a reasonable maximum output of 2 volts should be able to drive the amp to full power in conjunction with a zero gain preamp, on the peaks of recordings that have been engineered such that peaks reach levels that are close to "full scale." But having looked at actual signal waveforms of various recordings I can say that will sometimes not be the case. And in addition perhaps the OP is listening to LPs, it being very common of course for cartridge/phono stage combinations to provide much less than 2 volts on high volume peaks.
So my bottom line suspicion is that an active preamp is needed, or perhaps an integrated amp such as the Parasound Halo that was suggested (which provides a total gain of 38 db, 10 db in its preamp section and 28 db in its power amp section). But before recommending a course of action I would want to know what the source component(s) are.
Finally, regarding the flickering lights on the amp, the user manual states that is an indication that distortion is exceeding 1%, and will occur "occasionally" under high volume conditions. But if "they glow brightly or are on most of the time" ... "you are overdriving your equipment and should turn down your volume control."
Regards,
-- Al
