The Sovtek does have good bass. The Telefunken should provide equal or better, in addition to the improved highs you are already aware of. There is always the possibility that your units were designed around the Sovtek and both of them prefer the physical and electrical properties of same. The Sovtek is the only current production 12AX7 that is (IMO) really good.
The NOS tubes you mention will not react the same in every circuit, and unfortunately only trial and error will determine the best result. You should first be certain that the NOS tubes you are testing are not already worn out, and that they are free of microphonics. The other tube that has great bass is the Siemens CCa, and it too will react differently in different circuits. If in the end you find that the result with the NOS tubes are not what you want, just be happy with the Sovtek, it is indeed a bargain in today's tube market where the Telefunken and Siemens often command $50.00 and more per tube.
The NOS tubes you mention will not react the same in every circuit, and unfortunately only trial and error will determine the best result. You should first be certain that the NOS tubes you are testing are not already worn out, and that they are free of microphonics. The other tube that has great bass is the Siemens CCa, and it too will react differently in different circuits. If in the end you find that the result with the NOS tubes are not what you want, just be happy with the Sovtek, it is indeed a bargain in today's tube market where the Telefunken and Siemens often command $50.00 and more per tube.