And almarg has stated an important point about the use of the 5751 variant in your CJ preamp...
Please note: The indiscriminate substitution of type 12AX7 tubes for the 5751 will seriously degrade the sonic performance of the unit.
The 5751 has lower gain, so it's recommended to stay with a 12AX7.
This comment is backward. The book is saying not NOT to replace the 5751 with 12ax7. The higher gain of the 12ax7 would overdrive the circuit, being 30% higher. That’s why it says not to indiscriminately do that.