Do NOS Mullard EL34 tubes last much longer than NOS SED (Svetlana) EL34 ?

Yes … the REGA ISIS VALVE specs are :

This stage uses two military specification triple mica 5814A (ECC82/12AU7) triodes being driven by the revolutionary Wolfson WM8741 ultra high performance digital to analogue converters The output buffer and transformer driver stage uses two ECC88 (6DJ8/6922) triodes.

Regardless , IMO:

- any model brand new NOS MULLARDs should never crap out THIS friggin’ fast….AND ..,,

- the Commie bakeoff options did sound better in MY system :(REGA ISIS VALVE cdp/USB dac / REGA OSIRIS integrated amp / HARBETH M30,2 XD speakers / ATC subwoofer / all CARDAS CLEAR cabling loom )

I bought them from a NOS MULLARD established online specialty MULLARD dealer with a stellar reputation.I’m not blaming him.
Rather, It was my dumbass foolish arrogance to ignore an old school European trained professional tech’s recommendation to bypass the overhyped MULLARDs that grinds on me with my poor choice experience .. I won’t make the same mistake.

@akg_ca 

Why didn't the merchant just replace the tube?

I would have. No problem.

$450 for a fx1 is way to high for me.  I bought 4 quad’s of fx2 over time when I noticed prices going up years ago. I’ve been buying used xf2’s that test as new for 1/4 of the price for the fx1 quote you have. I’ve been running the same quad’s in a CJ amp for years with never an issue. I also have quad’s of sed wing C’s from the 80’s that to me do not sound near as good as the xf2’s. Some of the wing c’s have red plated on me, much better luck with the Mullard. 

New Tung Sol EL34B

I have used mullard xf2s, wing Cs, JJs, and a few others.  Then I found Gold Lion KT77s from upscale audio was the right choice for sound, reliability and price.