My terrible Customer Journey with the Line Magnetic LM-503PA Mono Vacuum Tube Amps

I am sorry to hear this and would never consider a product that self-destructs with a bad tube.   I would be concerned about an amp that overheated the way yours did.  The heat could cause expansion and damage to components, reducing the life of the product.    The manufacturer should send you a new matched pair of amplifiers and an apology, with a protection circuit for the tube issue.

Tubes can fail. Power tubes can fry transformer when this happens. We do not know if music was playing while it happens, but generally that sort of failure will cause loud noise in speaker, which is hard to miss even if you are not in the same room. Transformer does not burn out instantly - it takes at least few minutes for it to reach critical temperature when power tube is shorted (which is likely happened here). Some amplifiers have advanced protection from fault, but these will be higher tier brands like McIntosh or Audio Research. Generally I would not leave running tube power amplifier unattended.

When dealing with China based makers, I would not expect great customer service. In most cases it makes more sense to purchase product directly from China and forgo warranty support other than DOA. If device fails, use local independent repair shop to fix it. Or simply purchase a new one. In case of LM, price difference between ordering from China and EU distributor could allow getting a spare amplifier.

Amazing how many people are willing to abandon consideration of a brand based on a single customer's bad experience.  Oh, by the way, I had a bad experience with my Tesla....

Decided years ago to avoid China products, but not for all of the mostly faux altruistic reasons most pontificate about.  I simply do not like their engineering, mostly, and of course what you are experiencing.  Then again, try getting a piece of Sugden repaired in North America. No picnic there either.

Hindsight being 20/20, perhaps some Valvet monos would work well or some other European gear that provided real customer support.  Best of luck sorting out the whole mess.  We've mostly all been there.  It's unavoidable if you play the game enough.

Sad some companies are like this.
Just had a terrible experience with Cary Audio and an SLP-05 preamp, wound getting a refund.

Compare to my experience with Manley.
Before I purchased them (used still under warranty), I emailed them to verify the warranty was valid and asked a couple of questions at the same time.
I heard back from sales about the warranty, and got a personal email from EveAnna Herself.
After they arrived with tubes removed, I went to the website only to find they did not post a tube map - I had to zoom in on the image to figure out what tube went where. which was confounded by the fact that my input buffer tube was different than the one they listed.

So I emailed tech support to verify the tube was OK to use, and pointed out they might want to consider adding a tube map to the manual and website.
Got another personal email from Herself telling me that she just updated the website with text descriptions of the tube locations.

Some companies get it, some don't.