i have the stock bose system in my car and hooked up an amp and sub to it using the signal converters. there are some nights where i dont want to hear a lot of bass. i usually turn the bass down on the bose but then the interior speakers sound flat. is there such a product that i can use to control just the sub?

yup... you can get a bass knob from a car stereo shop...
 

for 30 dollars that plugs into your inputs that only controls the sub volume