tone control

[7"estatdef]

As Reece pointed out in the inexpensive sets the speaker is usually small thus limiting the bass response. There is another reason for this is as it will require better power supply filtering so that the 60cy hum is not heard. The small speaker allows manufacturers to use less filtering because the speaker isn't very effective at 60cy hence not much hum is heard.Bigger coupling caps aren't going to make much of a difference with a small speaker. With a bigger speaker and bigger coupling caps it will improve the bass response but may need to increase filtering to reduce hum. The older AC DC sets with electrodynamic speakers use their field coil as a choke in the power supply. This helps filter out the hum.
Terry