Too much $$$?
They make tubes that'll work in most of my gear, but I can buy nice NOS tubes for less. Also, the CV181 is supposed to be a 6SN7 but has a lower max plate voltage, requires 50% more heater current and has a mu of 32 vs 20 for a 6SN7 so they aren't necessarily a drop-in replacement, my preamp at 325V B+ might be too hot for the CV181...
As roscoe said, Psvane is distributing a new updated Treasure tube, the current production is old stock.