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