FFT spectra are usually crude. Even many apparent continuous displayed spectra are often based on small sample of some kind.
To really get down to the bare metal, you need to turn the oscillator knob by hand and take readings. Unfortunately, this is far easier said than done. At any given precise frequency, the level may fluctuate over a 20dB range just by moving a half meter or so. So which is the "correct" reading? (Here I can see, a warble tone generator would be very very nice...)
Anyway, I got the B&K Oscillator (cheap ugly but working) and Genrad 1933 meter (cost about as much as a car when new around 1973). I've done all my previous sweeping and tuning on the right channel. Now it's time to verify that I haven't messed things up too badly in the left.
Right now I'm playing 14Hz at the minimum output level of the B&K at gain 95 on the Tact.
14 Hz 80dB (effortless. speaker hardly moving)
16 Hz 85dB I can hear it
15 Hz 84dB barely audible
17 Hz 86dB
19-20 Hz 89dB some rattling, chuffing, or something
18 Hz 87dB
21 Hz 94dB vague noise, too loud
22 Hz 93dB too loud
30 85 sounds plenty loud
45 90 local peak
71 85 local peak
58 80 min, but similar all around here
78 60dB (ok, i tuned this by had to minimize the meter)