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.