I agree that the Chinese Shugang can sound terrific in that style circuit, especially for clean tones. I think Ruby brand uses alot of them. They werent my fav for natural overdrive, but not terrible either. The thing with new/Chinese tubes is they dont have the quality control of years past (US/Euro), so you may go thru a few before you find one that is superior. That is what the "selected" means, when you get the SMS that work has already been done for you. In other words, the first ShuGang you buy and install may not sound as good as what came with it bc they handpick the best out of a whole bunch before shipping the pre new. Chances you will get one are probably better than 50/50, perhaps not much though. Thats what alot of relabelers do today, they contract with the few producers to pick out certain spec tubes which they then can sell with confidence they couldnt otherwise.
If you want a heavier overdrive from your preamps the European and East Euro tubes can be great. You still have to go thru the work of selecting individual tubes with NOS/used USA/Euro's though bc they have been sitting around so long nobody knows whether they have been used or what. Most tube testers are not reliable beyond a point for this either bc they dont test the tube at specific higher voltages of your amp. I have had exc sounding tubes in one amp sound no good in different amps, perhaps due to voltage differences.
Most 12AX7's are either 15mm "shortplate" or 17mm "longplate". The longplate variety are more likely to be "microphonic". They can still sound fabulo0us microphonic, but you will have more background noise, clicks/pops, etc. The worst micros can hum and oscillate so much they are unusable for anybody. The longplate tend to have higher gain so alot of peeps like them for the 1st stage of preamp.
Output tubes may have an even larger affect on the tone of an amp, especially when played loudly.
