I own MA-1 mk3.1 upgraded to mk3.3 and MP-1 mk3.3. Once the MP-1 arrived, I quickly replaced the three fuses on the power supply unit. Ever since, I always complained to Ralph, his preamp hummed so badly. The phono stage also making noise. Ralph told me, it was very silent when they're testing the product at factory.
A few weeks went by, I couldn't stand the hum which immediately starts when I engaged the B+ on MA-1, I brought my MP-1 to a very reliable local tech.
He opened up the top plate and check everything. Nothing's wrong. Finally, he noticed that my fuse holders, those two small holders were not tight enough. So he helped me tightened it.
And my MP-1 is silent ever since. My bad, I was careless when replacing the fuse myself.
During those email and phone conversation with Ralph when I was frustrated with the hum, Ralph provided many feedback and instruction on how to check if any other my equipment caused the hum. Thanks to Ralph's patience and professionalism. He's a man with great passion for music and great with customer service as well. I stand with Ralph.