There's also Goldfinger mods - but to add bass?
The increase of caps values usually work.
The fidelity though - needs a mike that produces more bass to help with the "Sharpness" that the low-end crackle the bass sounds are (upper fundamentals)
The Value adds the caps provide - increase the bandwidth - and it's products - it's a two edged sword - to accomplish this - you have to let go of control of that - to accomplish it.
You can try to filter-in (as in make a few changes to help with the bandpass) but you'll get an instability issue and this puts you right back to having to increase the impedance values (lessen the signals strength or power - by increased resistor values) the larger value caps provides - reduction for signal input - which puts you right back near the center of the vocal bandpass the radio was set up for,
The biggest effort would be to change the 100uF to 220uF and the 473 to a 0.1 (104)uF might help (at AM Regulator)
There are other ones...
Just look for WWDX Handy Andy TRC-453 or WWDX Handy Andy PC-122 Mods - use Google - for some reason it works outside the WWDX forums - then Google can help direct you to the info you and your radio may need to have some sucess.
One being the audio chain to the Balanced Modulator once the Mic amp is done with amplifying what you're putting into it. Leave that alone as much as you can - for SSB audio will easily overload if your play with the Balanced modulator values