Do not remove any of the FOUR tubes in the phono section (the 4 on the left if you are looking down on the open unit from the front - controls near you, rca jacks away). To do so will alter the filament voltage and B+ as it is designed for a particular current draw. You will not hear the tubes so stick cheapies in the phono section and the good ones in the line. The phono section uses two 12AX7 tubes and one 5751 and 5965. The line stage uses three 5751 and one 5965. DO NOT MIX THESE UP. The 12AX7 is similar but different from the 5751 (mu of 100 and 70 respectivley amoungst other differences like plate resistance, etc...). The 5751 also draws a little more filament current. The 5965 is a medium mu tube and all together a different animal. It is closer to a 12AT7 but not the same.
Final comment. Use the preamp as it was designed.
Final comment. Use the preamp as it was designed.