First of all, thanks for providing the schematic. I get requests like yours but without schematics. Am I really expected to hunt down one? Second, I commend you for moving up out of the mere part-exchanger mentality into the higher realm of tube circuit understanding; very few audiophiles do. An acquaintance's single-ended tube amplifier disappointed him. So he bought the most expensive tubes he could find. It still disappoints. Its bandwidth begins to fall off at 8 kHz. And no amount of part exchanging is going to extend it. When told this, his reply was that was not what the people who sell expensive parts tell him (amazing, considering that those part sellers have not seen the circuit or even know how to use a scope.)
As for the line stage, you are right: it looks very cute. So cute that it would tempting to buy one and then gut all the original circuitry, as the circuit is about as unfortunate a design as I could imagine. A 12AX7 output stage??? Usually, we need only to look at the parts quality to see where an amplifier went wrong. But in this case, the part quality is relatively high. What went wrong was a poor circuit design. Just about any other circuit would be preferable and I can think of about five possibilities that would retain the same amount of tubes and many of the parts of the original. But first let's look into what is already there.
After a volume control, the signal passes to a cathode follower that then cascades into a grounded grid amplifier, which in turn, cascades into a cathode follower output stage. Another way of looking at the circuit is that it is made up of a common cathode amplifier cascading into a cathode follower.