The 5V4 is my favourite rectifier valve too - and by NOS standards, not yet that expensive.

Mullards GZ34s are getting very silly though, especially considering that as you say - being the highest quality and lowest resistance valve, are ironically the least valve-y sounding! I can hear the difference - I had a Mesa Maverick with a solid-state/valve rectifier switch so you could compare it on the fly - but only really when the amp was pushed a bit.