This one is slightly more advanced, but still easy to build for those with a little soldering experience. I didn't do an "EZ-Build" diagram - maybe I'll add that later.
Anyway, this is a simple Jfet based "booster" circuit. Instead of just making it a "clean boost", I expanded its use by adding the "Simple Varitone" circuit. You can find the Simple Varitone posted here on the site. What happens is that your signal goes into the Jfet (MPF102) and is amplified - then it goes out and variable frequencies are bled to ground (removed), unless you have the switch in the "open" position. In the "open" position, this circuit will act like a full range clean boost for your bass ! In the other positions, it can go from "just a bit of hi-cut" to "full on dub-reggae" bassy goodness !
A few things to keep in mind - you may need to adjust the value of the 10k resistor slightly to get the best sound. Certainly you could use a trim pot instead of the resistor, but I wanted to keep the project as simple as possible, without sacrificing tone and versatility. Use a 100k LOG pot for the volume pot. Lug 1 will connect to ground, lug 2 goes to output, and lug 3 from the circuit - hooked up just like a regular volume control.
I used a Jfet because it tends to, at least to my ears, sound better than a transistor based boost - at least on bass. You can use other Jfets besides the MPF102, like a J201 or 2N5457 ... but the MPF102 is easily available at Radio Shack as is the switch !), so in an effort to "keep it simple", I used the MPF102.
There are very few parts in this circuit, so it is ideal for a builder with minimal experience. You should, however, be familiar with the use of a soldering iron, how to hook up a battery to a circuit, etc before trying this project. There are a lot of great places to get info like this on the web, including www.diystompboxes.com
and many others.