All boomboxes have differences in input sensitivity and input impedances. The Sanyo M-X720 has an input impedance of 47k Ω, no sensitivity specified. Suggesting that all Hitachi's are the same is anything but the truth. That's because consider the following input impedance & sensitivity specs of these Hitachis:
TRK-8190, 8290 input sensitivity 100mV, 50k Ω
TRK-8600, J5 input sensitivity 200mV, 50k Ω
TRK-7620H input sensitivity 500mV, 680k Ω
3D-30, input impedance 700mv, 40kΩ
So as you can see, the impedances & sensitivity is all over the map. On the M-X720, the input impedance is 47kΩ & if you want to change a resistor, you can change R806 (& the matching one for the R channel), which coincidentally also happens to be.... 47kΩ. If you change that resistor, you might improve the sensitivity but you'll also change the input impedance. However, are you sure you want to alter the input impedance? It's already on the low side (see list above). The sensitivity of the inputs has to do with what the amplifier circuit does with the input signal, and some will have higher gains & some will have lower gains. Sanyo calibrated theirs for a particular reason. Also keep in mind that all of these circuits were designed for "real" line-level signals, not today's quasi earbud/line-out type circuits. The proper way to increase a systems sensitivity is to install an internal buffer amp to jack up the signal level or modify a circuit so that the amplification at that part of the circuit is increased. This would need to be custom designed for that particular circuit. Some service manuals, especially Pioneers, have a diagram called a level diagram. It shows you graphically the "level" of a signal as it travels through the circuit from input to output in DB's. Again, this is all designed and calibrated so that the signal is at a particular level at that particular point in a circuit.
Ideally, the best thing for you to do is get yourself a headphone amp. This will sit between the ipod & your boombox. Ideally, you'll get an adjustable one. That way, you can use it for any boombox and adjust as needed. User mellyselsr is high on and says really good things about ones made by FIIO.