Dear Bob: +++++ " You buy phono cartridges, pre-amps amps etc. that are flat 20-20k hz. So my audiophile goal is to get 20-20k hz flat ." +++++
Looking to your nice single item audio system IMHO it is almost impossible to obtain ( per se ) your desired goal, many factors in your audio system preclude it: your phono stage has an un-desired non-flat inverse RIAA eq. that it is greater than 0.1db from 20-20kHz, your phono cartridge has a deviation on the same frequency over 1.0db, you have severe deviations on your speaker response due to a high output impedance in the amplifiers, room interaction, etc, etc.
Of course that your goal is a desired one but alone could means nothing, that goal have to come along with very low distortions ( any kind ), noise, colorations, right tonal/natural balance, high resolution, etc, etc.
Now and speaking on your " same SPL " for every LP: I normally heard my system at the same SPL, SPL where my system is right on target where I can hear it for hours with out any fatigue " ear/brain " sign where I always enjoy what I'm hearing it does not matters what I'm hearing where my " foots " are always dancing where my brain and feelings are full of emotions.
Yes, there is one SPL for each audio system where everything is almost right but not necessary that fact means that every single LP should shine: NO!, there are several LPs that have its own and singular SPL ( different from what I hear normally ) where they are on target.
There are many examples about but one that comes to my memory is the Patricia Barber Cafe Blue 33rpm and 45rpm versions: many people say that the 45rpm version is not as good his 33rpm counterpart and that's is true if you want to hear both at the same volume but if you increment by 2-3db the SPL on the 45rpm everything comes on target.
So in the same way that every single audio system has its singular right SPL where it shine in the same way the LPs have its own SPL where it shine each one.
This means that if everyone of us always want to achieve the best ( about SPL ) on each one LP then we have to adjust the volume for each one LP where we can obtain the best natural tonal balance.
Now, what to do?, easy hear at the SPL where you are satisfied!!!!
Regards and enjoy the music.
Raul.