The amps I have built so far that did not use NOS tubes, I sourced basic JJ ECC83s/12AX7 tubes. I have been really happy with them, but when chasing that last little bit of hum, I wonder if I should have paid a little extra to get a low noise version. I see that JJ has the ECC803s which they say is lower noise and just a few $ more. There are others but you can get lost in the descriptions and the hype. Interested what advice others have for selection of preamp 12AX7s. If you go with low noise versions, what do you purchase and do you place them only on the input tube or do you also see an advantage using them deeper into the circuit?