Other Rectifier tubes Alternative for 5AR4

It’s not an issue in this case since you have directly heated cathodes, but the 5AR4 also has a controlled warmup time to prevent cathode stripping. At any rate there aren't any good subs for it.

Do you mean my amp which is based on the Luxman MQ-300 is directly heated cathodes, so, I need to use 5AR4 where it has a controlled warm up?  And I can’t use a 5R4 because it doesn’t have the controlled warm up?

No. I mean that in your amp that quality won't make a difference. Cathode stripping is a phenomena of indirectly heated power tubes. Your power tubes are directly heated (the filament is the cathode literally) so cathode stripping is impossible.

Late to this thread. Just today, I spoke with the authorized Luxman warranty tech, Mike Garry, about replacing the 5AR4 with a 5UG4 in the MQ-300. He said do not do it. The amp's huge power supplies have an inrush that could be too much for a 5UG4. Or something like that. Glad I asked because I just bought an MQ-300 and I have a stash of nice 5UG4 that I would have tried in it.

@wrm57 
thank you very much.  

Can it be confirmed that the LS100 will accommodate a coke bottle 5u4g in height inside the unit?