That “getting louder and louder” thing is all about feedback. You strike the string. It sends a signal through your amplifier. That signal comes out your speaker as sound. That sound hits your string — which is already vibrating at 130 Hz — with its own 130 Hz sound. That makes your string vibrate more strongly, which puts a stronger signal into your amplifier, which makes your speakers louder, which hits your string harder and makes it vibrate even more strongly, which puts an even stronger signal into your amp, which makes your speaker louder still, which hits your string even more strongly... do you see the pattern?
That feedback loop is affected by any losses or gains that occur. At a room resonant frequency, gain is added to the feedback loop, and the feedback gets stronger. If you turn down your volume knob, there is a loss, and the feedback gets weaker. If you add sound-absorbing material, there is a loss, and the feedback gets weaker. With enough losses in the feedback loop, the feedback disappears altogether.
