Tubes for a pre stage... matched NEW or unmatched identical NOS?

i would always go for matched sets when they appear in serious hifi gear

of course testing does not guarantee the electrical match seen on the tester will persist over the lifetime of the tubes in use, but it is a much better starting point than starting with wildly mismatched ones at the get-go

that said, i agree with others who have said that old stock 12au7’s are still plentiful, and not very expensive

i have over a hundred of them in matched sets collected since the 80’s, mostly american made ones (sylvania, rca, ge and so on) ... happy to assist if anyone here (in the usa) needs some

Use matched sets. I f not you'll be listening, thinking how good it sounds, then you'll wonder how it would have sounded with matched tubes.

@tomcy6 - Bob does some great work. He used to do Tube Only repair and customization and has his own patented circuit designs. He lives a mile away and has done some repairs for me even after he was building the new Pres. It's a nice sight seeing 6-8 of them on the bench in progress.

On my Audio Research SP6B, the individual tubes can be biased, and it definitively improves sound quality. 
 

+1 re Brent Jessee, excellent guy, my matched Raytheon VT231’s and CBS/Hytron JAN 6S7N’s are wonderful, and he was a knowledgeable pleasure to work with in tube selection. 

Most preamp circuits are run class A due to low signal levels. You want them matched for gain and screened for low noise, again because the signals are low at this stage.