If you have an oscilloscope and a function generator or an audio oscillator, you can inject a 10Hz sine wave into the input of the preamp and follow it through the circuit. It's going to be hard without a schematic that would let you know how much gain to expect through each stage, but at least you could see if it is an easy modification or not.
If it were me, I'd inject a 10Hz signal into, say, the Aux input of either channel. Set your tone controls (if any) to flat. Set the volume control to max, then find the coupling capacitors that couple each gain stage to the next. Measure the signal on each side of the cap and see if it drops after the cap. Basically, I'd be looking for an R/C combo that is a high pass filter, and then do the math to lower the 3dB knee to 5 or 10 Hz.
If it were me, I'd inject a 10Hz signal into, say, the Aux input of either channel. Set your tone controls (if any) to flat. Set the volume control to max, then find the coupling capacitors that couple each gain stage to the next. Measure the signal on each side of the cap and see if it drops after the cap. Basically, I'd be looking for an R/C combo that is a high pass filter, and then do the math to lower the 3dB knee to 5 or 10 Hz.
