Other Rectifier tubes Alternative for 5AR4

The 5AR4 has a very low forward voltage drop for a given plate current. Substituting another rectifier may likely  cause a  B+ voltage sag which may affect performance. That’s probably why Luxman doesn’t give you other choices. Safety wise it’s not an issue.

Use what the circuit was designed with! Different rectifier tubes have different voltage drops and limits on current draw. Plus different current draws for the heaters - which can overheat power transformers.

For instance, the 5Y3 (aka the 80) has much less current handling than a 5AR4 (GZ34) or a 5U4. It will quickly red plate (overheat) if used as a substitute!

I'm not sure what other rectifier tubes are a safe replacement for 5ARA but when I played around with different vintages of 5ARA I found the metal base tubes (mid 1950's) made the largest positive change to my SQ than any other charge I had previously made.  They are expensive but I would try one before any other changes.  By the way I've used them with ModWright pre, modded Sony CD and Bob Latino VTA ST-70 which I build.    

5ar4 is by a long stretch the best tube rectifier for audio in my experience... all other common alternatives are weaker and less durable

get fresh strong old stock brit euro or usa made ones and be done for this lifetime