Do small preamp tubes 12ax7 need to be matched?

The only small signal tubes I closely match are ones used as long-tailed phase invertors. Most amps don't have any provision to adjust the AC balance of the driver stage. For that I use a VTV small signal tube tester. It also lets you listen to the tube so you can check for noise and microphonics. Very helpful if you need to screen alot of tubes.

I always use matched tubes. Many amps allow output tubes to be biased individually but I have never seen that for smaller tubes.

Note: Tubes like the 12AU7 are stereo tubes, i.e. they each have a pair of plates, grids and cathodes. I am unfamiliar with the circuitry of the Aesthetix but many of the preamps and phono amps I have built and modified I found some tubes where half of the tube is used for one channel, the other half of the tube for the other channel. So if you are really concerned about matching tubes (like I am), make sure that the tube is matched with itself when a single tube is used as amplification stage for both channels.

I buy from a reputable dealer who always sends me matched tubes at no extra cost.

Regards
Paul

How can you tell if the tubes ate matched?  If you have the same brand etc.. are they likely matched?

I had a Telefunken 12ax7 tube go out on me in the V4 (R) position of my Aesthetix Rhea, so I replaced the pair with stock Sovtek 12ax7WBs. Do I need to replace the Teles with a matched pair, or can I just pick up a single and throw it in.

You can also use 12AT7 (ECC801s) instead of 12AX7 , military Telefunken tubes (if they are not fake) are much better than Russian Sovtek or any other new tubes.

Ideally yes but you don’t need to be too OCD - with small triodes, often you’re trying to match 4 elements (2 triodes per tube) so it can be frustrating to try to find 4 within 10% of each other on the major parameters (mu, transconductance) - especially vintage tubes. And even up to a 20% difference sounds like a lot, but it can be just fine for many applications. If your elements are within 15% (max) of each other you’re generally doing quite well. I believe Upscale matches their "Platinum" grade to within 10%.

If you have an egregious mismatch, well over 25%, then that can start to cause very audible issues. I had a pair of Cifte 6189 silver plates where one tube was much stronger than the other, and it caused about a 1 dB L/R channel imbalance when used in the mu follower slots of my phono stage.

Once I also bought a trio (pair and a spare) of Brimar CV4004 (12ax7) from Parts Connexion, unaware that they were going to be COMPLETELY unmatched from an extremely variable stock. They were a complete mess, egregiously mismatched on triodes and tubes (variances or 40% and more), and sounded like an absolute mess as a phase splitter my tube amps. Fortunately the vast majority of dual-triode tubes that have been sold to me as "matched pairs", over the years, have been as advertised and sound symmetric when in an amp. Parts Connexion is the only shop/dealer I'll never buy tubes from again. 

Some of the old tubes were abused by gear, more on one side than the other. I’ve seen a number of Tung-Sol BG RP 6SN7 (the holy grail tube) where one triode measures like new and the other is practically dead (measuring 50% less than the strong triode). Those are no good, so watch for that :(