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

I’ve never seen that option on preamp tubes! Have you?

From Backert Labs, who make only preamps:

Our Rhythm, Rhumba Extreme, and Rhumba Xphono preamps automatically and continuously adjust the bias that is applied to your tubes, ensuring correct bias at all times.

Adjustment is automatic, requiring no action on the user’s part.

Why is this important? Each pair of tubes requires slightly different bias. The result of our automatic bias adjustment: a circuit that brings out the very best from every tube you insert.

So Preamps Require Tube Biasing?

Actually, no. None of the other tube preamps on the market today, as far as we know, adjusts bias. And they work just fine. In fact our own Rhumba preamp does not adjust tube bias.  And it sounds great. This is because the tube bias, set at the factory, works perfectly with the stock tubes that we supply.

When bias adjustment matters is when you experiment with other brands of 12au7 tubes. Or tubes that are compatible with 12au7, such as 5814A, or CV4003. Some of these will have unusual bias requirements, which can make them sound “not their best” if those unusual bias requirements are not met.

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.