Not often, actually. That might be OK to assume for the purposes at hand, but if one checks out population spread for the purposes of designing for audibility it's another story - quite scary really - plus the effects of 'ageing' begin at pretty young ages. So IMO 20kHz is generally a safe overstatement. I guess most of us have experienced the wake up call when the VU meter's moving but can't hear anything during calibration tests, first happened to me in my mid 30s with a R2R test tape.......and seems that's normal.
The first grid now sees a different impedance. If the circuit layout was marginal then this might tip it over the edge.
Some DMMs have a frequency range, but not many. Can you hear the oscillation? If so, what does it sound like? If you can't hear it how do you know it is oscillating?
Yes, mine has a frequency range - where do I place the probes to test? I can hear it oscillating when I hook up an old speaker, sounds like a distorted radio station. If I power off, the noise lingers for a while. This is obviously with no input connected.
Spice should show this roll off, try experimenting and see what happens with the simulation.
You need to short the input for testing, or else high gain stages can start to oscillate. Just clip a jumper from the tip of jack to ground. When the volume pot was in it was shorting the input to ground(when turned down of course), hence no oscillation.
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