I own the Schiit Saga Preamp and the Adcom GFA-5800 amp.
There is either something wrong with your GFA-555 amp or maybe the source. The GFA-555 is plenty powerful to drive the MMGs to a loud level.
You did not mention your source of music. If you are using a turntable, the Schiit Saga preamp does not have a phono input. It is all line level. You need to use a phono preamplifier between the turntable and preamp.
If you have a CD player, you should connect that to your preamp to see if anything has changed.
There is either something wrong with your GFA-555 amp or maybe the source. The GFA-555 is plenty powerful to drive the MMGs to a loud level.
You did not mention your source of music. If you are using a turntable, the Schiit Saga preamp does not have a phono input. It is all line level. You need to use a phono preamplifier between the turntable and preamp.
If you have a CD player, you should connect that to your preamp to see if anything has changed.