Here is a copied and pasted (and somewhat modified) answer to this question from one of my older posts here at Audiogon.
The first four 12AX7 tubes (near the input jacks) are VERY critical, and any noise or microphonics in that position are going to make your life miserable. If you get professional help with any tube selection at all, make it these four.
Other tubes that make a big difference is to replace the two Sovtek 6SN7 with the RCA 5692. The old military red base 5692 is the best, but the black plate RCA 6SN7 is a great choice as well.
The Sovtek 6922 tubes used in the output of the Io are so bad, that almost any brand is better. I like the Telefunken 6DJ8, the Mullard 6922 white dot, and Amperex 7308 all nearly equal. The sound of these three sets of tubes is radically different, and the correct choice is in the ears of the beholder.
The Sovtek 12AX7 tubes used in the outboard power supply, while audible, are not a good place to begin. Going to something extreme like the Telefunken 12AX7 ( as I have ) will be audible, however everything else needs to be at the limit, for this to pay off. Changing the EL34 tubes right beside the 12AX7's (still speaking of power supply) have a large effect on the sound. The two best are the MO Valve KT66 and the Mullard EL34. I have tried almost every tube that will plug in that socket, and these two are by far the best.
At one point , I was running my Io 24 hours a day, NEVER turning it off. I pulled the Telefunken 12AX7 and MO Valve KT66 tubes from the two power supplies, and tested them against the recorded numbers from the year before. The loss in trans conductance was only about 12%. At this rate, the power supplies should run 24 / 7 for about 4 or 5 years. Upon testing the main unit, the numbers showed the 5692 (replacement for 6SN7) at about 15% loss in the first year, and the Mullard 6922 white dots (only about 6 months old now) show no loss at all. The main problem with this unit is the fact that ALL the gain is with 9 pin miniature tubes. There are no MOSfets or transistors at all to do this amplification. To make matters more volatile, the circuit uses NO feedback and is run with extremely high gain. This unit will take the .5 MV signal from a moving coil cartridge, and drive 40 feet of interconnect to amps requiring 5 volts peak to peak input signal. There is not another product requiring so much from the first few tubes in a step up stage. This simply means that some people are going to have terrible luck with plugging in tubes (especially UNTESTED) and others will have no problem at all. I think this unit fares pretty well as far as maintenance, but with the small signals from your phono cartridge being magnified with microscope like gain, care must be taken. If you have a problem or make a mistake, it will show up. The good news is that if this phono amplifier is set up correctly, it is without question the best product of it's type I have ever heard.
As to order of preference, the 6922 Sovtek would be my first change. For additional midrange and air, try the Siemens 6922 CCa or the Amperex US Military 7308 CEP. If either of these produce too much midrange presence and lack of emotional involvement, try the Mullard 6922 (gold pin). Realize, that when you upgrade to one of these higher resolution NOS 6922's you are listening with more intensity to the weaknesses of the remaining (original) tubes. You are judging the replacement, plus all that it exposes downstream.
If after changing to the CCa or CEP, you find that there is too much "grit" in the sound, try swapping the 6SN-7 as described in my long post. After that, if you are feeling adventuresome, trade out the first stage 12AX7 tubes (Io only). Note, these 12AX7's are the ones I warned about, get TESTED, ultra noise tubes for this position. My FIRST choice and the most beautiful sound is the Telefunken 12AX7.
Other options are the RCA 5751, a US Military version of the 12AX7. The 5751 will alter the tonal balance somewhat as it is NOT a match electrically. In addition to sounding dry, there will be a slight loss in gain, with slightly improved signal to noise.
One final suggestion would be the 12AX7 French Mazda. A good sample of this will produce significant improvements in high frequency transparency, especially compared to the Sovtek. However, the Mazda has not been without problems in my experience. The samples I received are not equally reliable or equally low noise. Some will play perfectly for many months while retaining their original "voice" while other samples will develop horrible microphonics and tube noise in as little as a week. This is a situation that could lead to disappointment with the Io, when in fact, the tubes are the culprit. Unfortunately, there must be some risk and experimentation within your own system to determine what is best.
The first four 12AX7 tubes (near the input jacks) are VERY critical, and any noise or microphonics in that position are going to make your life miserable. If you get professional help with any tube selection at all, make it these four.
Other tubes that make a big difference is to replace the two Sovtek 6SN7 with the RCA 5692. The old military red base 5692 is the best, but the black plate RCA 6SN7 is a great choice as well.
The Sovtek 6922 tubes used in the output of the Io are so bad, that almost any brand is better. I like the Telefunken 6DJ8, the Mullard 6922 white dot, and Amperex 7308 all nearly equal. The sound of these three sets of tubes is radically different, and the correct choice is in the ears of the beholder.
The Sovtek 12AX7 tubes used in the outboard power supply, while audible, are not a good place to begin. Going to something extreme like the Telefunken 12AX7 ( as I have ) will be audible, however everything else needs to be at the limit, for this to pay off. Changing the EL34 tubes right beside the 12AX7's (still speaking of power supply) have a large effect on the sound. The two best are the MO Valve KT66 and the Mullard EL34. I have tried almost every tube that will plug in that socket, and these two are by far the best.
At one point , I was running my Io 24 hours a day, NEVER turning it off. I pulled the Telefunken 12AX7 and MO Valve KT66 tubes from the two power supplies, and tested them against the recorded numbers from the year before. The loss in trans conductance was only about 12%. At this rate, the power supplies should run 24 / 7 for about 4 or 5 years. Upon testing the main unit, the numbers showed the 5692 (replacement for 6SN7) at about 15% loss in the first year, and the Mullard 6922 white dots (only about 6 months old now) show no loss at all. The main problem with this unit is the fact that ALL the gain is with 9 pin miniature tubes. There are no MOSfets or transistors at all to do this amplification. To make matters more volatile, the circuit uses NO feedback and is run with extremely high gain. This unit will take the .5 MV signal from a moving coil cartridge, and drive 40 feet of interconnect to amps requiring 5 volts peak to peak input signal. There is not another product requiring so much from the first few tubes in a step up stage. This simply means that some people are going to have terrible luck with plugging in tubes (especially UNTESTED) and others will have no problem at all. I think this unit fares pretty well as far as maintenance, but with the small signals from your phono cartridge being magnified with microscope like gain, care must be taken. If you have a problem or make a mistake, it will show up. The good news is that if this phono amplifier is set up correctly, it is without question the best product of it's type I have ever heard.
As to order of preference, the 6922 Sovtek would be my first change. For additional midrange and air, try the Siemens 6922 CCa or the Amperex US Military 7308 CEP. If either of these produce too much midrange presence and lack of emotional involvement, try the Mullard 6922 (gold pin). Realize, that when you upgrade to one of these higher resolution NOS 6922's you are listening with more intensity to the weaknesses of the remaining (original) tubes. You are judging the replacement, plus all that it exposes downstream.
If after changing to the CCa or CEP, you find that there is too much "grit" in the sound, try swapping the 6SN-7 as described in my long post. After that, if you are feeling adventuresome, trade out the first stage 12AX7 tubes (Io only). Note, these 12AX7's are the ones I warned about, get TESTED, ultra noise tubes for this position. My FIRST choice and the most beautiful sound is the Telefunken 12AX7.
Other options are the RCA 5751, a US Military version of the 12AX7. The 5751 will alter the tonal balance somewhat as it is NOT a match electrically. In addition to sounding dry, there will be a slight loss in gain, with slightly improved signal to noise.
One final suggestion would be the 12AX7 French Mazda. A good sample of this will produce significant improvements in high frequency transparency, especially compared to the Sovtek. However, the Mazda has not been without problems in my experience. The samples I received are not equally reliable or equally low noise. Some will play perfectly for many months while retaining their original "voice" while other samples will develop horrible microphonics and tube noise in as little as a week. This is a situation that could lead to disappointment with the Io, when in fact, the tubes are the culprit. Unfortunately, there must be some risk and experimentation within your own system to determine what is best.