Are you operating in the correct SPL window for high-fidelity listening?

markalarsen, You wrote, "The differences cannot be so substantial that they cannot be compared."

I use the same meter as you, the Radio Shack. I listen to full range ESL speakers. When I take measurements from my listening position, the readings vary by at least 5-10 db depending exquisitely on how I point the meter.  That is the very thing I am talking about.  And that result is using the RS meter.  How would those results compare to using the iPhone meter or some other meter or if the weighting was different, without my altering any other test condition? (A weighting gives different results from C weighting with the RS meter, too.) The variability in my results using the same LP and the same volume settings on my preamp ranges the same as what others refer to as "different" or a diference in their preferences compared to another contributor. This is not meaningful.

I feel like SPL is both mood- and recording-dependent. 

Sometimes I’ll listen to 100 db sludge metal while performing certain tasks because it helps me focus. 

Other times, I am content listening to sixties big band at volumes that don’t intrude on quiet conversation.

A good system should accommodate both without significant loss of sound quality or resolution. I find that the upper limit beyond which listening becomes uncomfortable is determined more by the room than by the system, in my case that is. 

Preamplification / attenuation is a topic unto itself that has not been satisfactory explained in this thread so far, and though not the main topic it is inextricably linked. Suffice it to say that a very good volume control can make a significant improvement in sound quality, while a mediocre one (which is to say the vast majority) has the ability to ruin your low-SPL listening experience.

This thread reminds me of conversations I have had with employees. I would give them an assignment. They would return and tell me it could not be done. I told them I was not interested in the ten ways it could not be done but only in the one way it could be done. 90% of the time they then returned with a solution. 

Garbage in, garbage out.

If you want meaningful data, stipulate a method for making the measurement and with what device. For example:

from listening position

meter at head height aimed at a point mid way between stereo speakers

state nature of the music

what meter

A or C weighted?

room size