Do small preamp tubes 12ax7 need to be 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 :(

Just recently I purchased two early 1960’s E88CC Tubes and two Genalex Gold Lion E88CC Tubes, both sets were advertised as matched pairs.

I thought I was buying a safeguard tube with the Genalex to tie me over whilst I was awaiting the usable vintage valves to be discovered.
On testing both sets of valves to Vintage measured as good as new.
One Genalex would not produce a reading.

I personally like to have a matched measured valves to use.

Matched tubes will always give better sound.

You can "use" a 12AT7 to replace a 12AX7, in that the pin connections of the two tube types are identical, but that would be far from ideal.  The 12AT7 has less gain, a lower plate resistance, and a higher transconductance compared to a 12AX7.  In a phono stage, the difference in gain would be quite noticeable, even with high output cartridges, if one were to replace a 12AX7 at the phono input with a 12AT7.  But also the 12AT7 would function suboptimally in a circuit designed for a 12AX7, so there might also be noticeable distortion.  (I am qualifying my statements, because I know someone is going to claim that he used a 12AT7 in this fashion and it sounded wonderful.)

Mulveling mentions encountering 6SN7s where one triode tests like new and the other triode tests like its near death.  (The 6SN7 is a dual triode, with two completely independent triodes in a single glass envelope.) This can happen with a tube that has been pulled from a piece of equipment where only one of the two triodes (the dead one) was actually attached to the circuit.   Often equipment was built this way so that one could swap L and R channel tubes after the tubes seemed to need replacing, and thereby gain use of the unused section in the opposite channel.  

I am not a big believer in paying a premium for matched tubes, because tubes do not age in synchrony.  So after a few months of use, one of those two tubes may test quite differently from the other.