Hi Rather nice build. It is interesting that the layout of the phono section is really quite close to the same as mine. Some things just work that way. It is toward the front in the photos of the insides of my integrated preamp (digital remote, line stage and phono sections). The PS and actual inputs are under the screens. Since the photo I have upgraded all the coupling caps to Auricaps and Jantzen Silver Z caps. The RIAA network is on a plug in so I can fiddle with it easily. The DIP switches allow for different values of phono cartridge loading. Yes tube rolling the input tube will alter the RIAA curve. I find that generally it shifts the whole curve up or down in frequency, but generally maintains the shape. The value of the first resistor is based on the anode resistance of the first stage and the input resistor of the second. with a 470K on the second grid to ground and a grid stopper of 22K, the value of the resistor is 89K minus the anode resistance of the SRPP. With most 12AX7 types and brands this will put it in the range of 68K to 73K. With the JJ ECC83 Gold pin tubes it works out to just under 71K.
Pre inisde with covers2.jpg
I did not add in the additional time constant as measurements indicated it was not needed. Adherence to the standard was at the limits of may ability to measure accurately. (BTW, the best way I found to do this was with a DSO in spectrum analyzer mode).