This is an old thread and the OP is in Germany. But I do want to share my experience with my Line Magnetic LM-523 PA. This is a different amp from the LM 503 PA mentioned in the OP. The LM-523PA is a pair of mono blocks. The tube compliments are for each amp 1x 805 output tubes driven by 1 x 300b. There is a 274b rectifier, and a 6sn7 and a ECC82. The amps put out 50w as oppose to 24 w of the LM-503.
I live in Taiwan. One would think with Taiwan’s proximity to China, that Chinese hi-fi brands should be abundantly represented in Taiwan. But for political and cultural reasons which I will not get into, there are no distributors for Line Magnetic, or any other Chinese hi-fi brands here. In order to buy, I would have to find an audio dealer who is willing to import items of interest from abroad, most likely from Hong Kong, if not directly from China. The question of warranty then is up to the dealer to honor.
I asked a dealer who I recently bought a Kinky Audio Tai Heng power conditioner to assist in acquiring a LM-523 PA. He is a rather resourceful young man, and gave me a quotation within an hour. Within a few days, I placed an order with him. And within two weeks the amps were delivered to my home. They were well packaged with paper boxes. I played the amps for a good two weeks without any problems. The tubes were all factory provided stock tubes.
One evening past the two week period, as I turned on one of the amps, a burnt smell filled the room. I immediately turned off the amps. In order to find out what was going on, I had to turn them back on. And on this second try, a 300b tube on one of the amps turned pruple haze. I took videos, then shut down the amps. I sent the videos to my dealer who then sent them directly to someone in Line Magnetic. Within an hour of back and forth, it was established that thwe problem was a bad 300b. Line Magnetic would send a replacement the next day. Within a week I received the replacement tube. I decided to put the other good 300b that I had as a replacement. And the new tube Line Magnetic sent I put in the other amp. As soon as I turned the amps on, the same purple haze appeared in the same spot as the previous 300b slot. Again I took videos. This time a technician at Line Magentic wanted me to open up the one amp where two 300b went bad. Long story short, it was determined that the second stock tube was the cause, and another replacement would be sent.
My point is that throughout these two incidents, I literally got in touch with Line Magnetic within 30 minutes through my dealer. And within a week new tubes were sent. I was lucky that the amps themselves were not the cause of the problems. I would say that compared to 2023, the QC by 2026 has improved on the production of the amps themselves. Even having both stock 300b tubes go bad surpriced the people at Line Magnetic. They claimed that has never happened before.
It is obvious that my circumstance is very different from the OP’s. I live in the same time zone as China. Buit the prompt resposes that I received at 9:00 pm - 10:00pm in the evening surprised not just myself, but the dealer who I bought the amps from. He told me that He initially wanted to purchase these amps from other reputable Hong Kong dealers. But as soon as they realize that we wanted 110v models, he had to deal with the factory directly. And He had never dealt with Line Magnetic before.
It has been another month from the above mentioned incident. The amps may yet cause problems. And if they do, the technician did say that they would provide support to a local tech to fix the problem, rather then hae me send them back to China. Therefore the risk of dealing with repairs is still a potential problem. But so far, the people at Line Magnetic have been standing up to their responsibilities!