New OP-AMP for Music Man HD-130 bass amp

I have several questions for the opamp experts about updating a 1974 MusicMan Bass Guitar head. The stock opamps in the preamp are obsolete LM 307H's. These are some of the first opamps used in music equiptment and since this amp was given to my friend for free I decided to give it a update. The output and driver is tube also. EL34's (4) for the output and 12AX7 driver. I found a way to install OPA 627's with a socket adapter from digi-key and I made a new power supply for the opamp section from a Jung-Childress design and I now want to know if I should change the topology such as removing coupling caps between stages or just increasing values of capacitance.I can't figure out what the two diodes in parallel with the inverting and non-inverting input of the first opamp do.They are reversed in polarity with each other. Input clipping protection? I can give more detail on this circuit if needed. I forgot to mention it is 3 opamps or stages and one is for tone control(bass,mid,treble). Any and all comments or suggestions would be most welcome.