Tube selection

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.

Albertporter makes a good point. I have a Siemans 12ax7 in combination with 12au7s made by Amperex in my Anthem amp. I like this combo,but on the flip side the Bugle Boys in my dac(sonicfrontiers) sounded soft compared the the Sovteks I replaced them with. Then again the BB's probably had some age on them.(they are used) Good listening!

I replaced Sovtek 12AX7's with Telefunkens.These go in the first 2 positions of my Rogue 99 phono stage.The Tels are much quieter and offer better bass,highs and more liquid mids.The price sure is much higher than the Sovteks but the sovteks have been known to have a short life span.The Tels are good for 10-100,000 hours.Sounds like a bargain to me!

As a designer of Professional Broadcast Audio Equipment, I am surprised about the discussions about Tube Sound. The characteristics of individual Tubes and Transistors of the same type vary by a very considerable margin. In good design, it is not good practise to rely on the specified characteristics of a particular active component such as for example a Tube. Instead, the design should allow for wide variations in gain, and be arranged to compensate for this. I have recently designed a Professional Valve Power Amplifier for serious Monitoring using ECC82/12AU7 in the driver circuitry. The design use generous Cathode Degeneration. Ie. what is known in solid state Amplifiers as nested or local feedback. Testing about 25 ECC82/12AU7 Tubes of various makes and ages, some used, I found no significant differences in overall performance of the amplifier, no doubt thanks to the way the Tubes were applied. An Amplifier relying on precise performance characteristics of Tubes or Transistors for that matter is badly designed. The Professonal Amplifier mentioned use either KT88 or 6550 Tubes in its output stages. There is practically no measured difference in performance between KT88 and 6550 Tubes except for overload behaviour. As expected, the 6550 Tubes produce marginally more power and overload more abruptly than the KT88 with the KT88's sonicly being more pleasant because of their softer overload behaviour. This is understandable and as expected considering Beam tetrodes versus Pentodes. I can see no reason for differences in sound quality between healthy Tubes used in low level stages other than poor or bad design of the circuitry in the first place.