PRR is giving examples of acceptable enclosure volumes. He is comparing a 4 liter rectangular box to a piece of tube with the same internal volume to indicate that both are reasonable test enclosures for your sub.
Typically you can connect a modern device’s output to multiple inputs if both devices are high impedance. Two amp inputs would probably not strain a cell phone’s output jack, especially given that those jacks can drive headphones. You should assume that the lower the load the higher the distortion.