Luxman L-550ax vs. L-350 AII

The L-550ax is made in Japan and the L-350II is made in China.  There's your difference in cost.

Made in China but fully backed by (and part of) Luxman is fine with me.  What about the sound?  Are they comparable?

I found this on the Luxman distributor's web site:

"The Luxman L-350AII is a rebadged L-550AII with all the critical Japanese audio parts, sourced from Luxman in Japan but assembled in China. As a result we were able to lower the retail price from $5000 to $3500 – a 30% decrease".

Unfortunately, it does not answer your question about the sound.  I suggest you call On higher Note and ask them.  Please see:

https://onahighernote.com