Is bass the most important frequency band?

This is my opinion only. And I got no science or DSP to prove my point. Only lots of trial and error in lots of rooms.

Bass in not the most important.
It’s just the easiest for us to quantify. When bass is full and clean we think we nailed it.

With my last house, I realized that this can lead you to stop pursuing improvement.
A lot of what we don’t hear is time smear. It’s tougher to hear at low volumes and as we move the volume knob up the smear increases smoothly. So it's tough to quantify. Bass can seem fine, but it too will improve when smear is reduced.

Handling first reflection points help but I considered that ’stage one’ only and just a starting point.

My wife moved my record rack right up against the left speaker. Much too close to be the ideal first reflection point. A lot of things snapped into focus. The record rack was 3 tiers high so it was at tweeter height.

It looked wrong but it was doing something right. It made me experiment a lot more with the room. I was in component buying mode at the time and I briefly put a pause on it as I got obsessed with the room.

A year later, the room was 180 different. Rack was on the side. Nothing between speakers...nothing but carpet. Speakers alone on carpet. Was it ideal? No, because it was a dining room...so there was a big Danish dining room table in middle. But it sounded sublime.

Bass was less full than before but way deeper than it had a right to be. No subwoofer. I now had sub bass coming from Salk Songtowers with 5" woofers. Not on every track but when I got a good record on...holy moly. Sound was supremely holographic. Could it have been better? Sure. But it’s the best I ever had.

Time smear is always there because there is a room to deal with. Deal with smear that’s closest...that will give you clarity. Time smear at further intervals makes things sound more spacious. So it’s a tough nut to crack.

Here’s a generality: treat the whole area around the speakers with the utmost care and attention. Right up to the room corners. The rest of the space is important too but that area is sacred.

Don’t let good bass stop you from finding better sound. There is always a way to make something work and you don’t need purpose-made acoustic panels in your living room...just get creative.

Notice that the bass and drums are usually called the rhythm section. As to whether the bass is the most important, I'm not sure that is proper. However It is the back bone of the music which is built upon it. This is especially true with R&R.

Yes, you can measure bass. In order to get good bass a measurement microphone and something like REW helps a lot.

I use REW with a calibrated microphone. The question wasn't meant to be if we could measure bass, but if we could measure the goodness of bass. In other words, can we make measurements that tell us if the bass is likely to be good or bad? It's a rhetorical question because I think we can, to some degree. I'm wondering what anybody here may think qualifies as good bass in acoustical terms. What measurement criteria correlate strongly with the perception of excellent bass?

But to answer your question, that would be a flat response measurement,  as close as one can…

I agree with flat response, but also think group delay, and appropriately fast decay times are important, along with adequately low noise and distortion. There's a lot written up about what kinds of numbers there should be for these measurements and it does seem to get a bit complicated. Plus or minus 3dB comes to mind for flat response, but really narrow band peaks and dips can exceed that and still sound good, while really wide band anomalies might have to stay within less than 1dB to sound good.

Here's a paper on time domain issues with bass: https://assets.ctfassets.net/4zjnzn055a4v/5bzV3RQ7DwopgxW3Joc84i/8ba19fed08c24feb61dc8ddbdc120b0e/20...

"The main conclusion is that at typical listening levels down to 100 Hz  the modal decay time T60 is allowed to increase from about .3 seconds by .1  to .4 seconds, while at 50 Hz even decay times of up to 2 seconds do not make a noticeable difference." 

Keep in mind that they were EQing resonance frequencies down to make their perceived level the same in attempt to isolate just the perception of decay time, suggesting that to not notice the long decay times you will need an equalizer.