The timbre control is just a tone control that controls the highest frequencies and the squelch sensitivity is for quieting the noise as you tune between stations just like a squelch control on a two way radio. Do you have the hum with the volume turned all the way down? Do you have the hum on all functions? If the problem is the filter capacitors the hum will be present with the volume turned down. The replacement filters you get now will be physically much smaller than the originals, I've been buying nearly all my electrolytic capacitors from
www.mouser.com they do not have a minimum order and the prices are very competitive.