Tube Distortion Pedal DIY **Need Help**

I've been researching a bit into tube amps for awhile (if I get one, I'm building a simple 5W myself first). I've gotten a wonderful idea, but can't really find where someone has done this yet, or any info on how to do it. I understand the basics of the circuitry, but not quite sure what every component is doing.

So, I'm guessing guitar would be easiest for this. I ultimately want to build 2 of these, one for Guitar, and one for my Bass. I figure I can tweak cap and resistor values for the bass after I build one for the guitar first.

What I want is a low power (9-12 V(DC?) basic standard pedal voltages) tube pedal to create saturation, and be able to switch into an overdrive (heavy for the guitar... think close to an early 90's Marshall in the hands of a death metal band). I play through SS combos or direct into the PA, so I'm always going to be running the cleanest possible in the pre and amp stages, and leave the effects within the pedal chain.

Anyone know how to do this? From what I've seen for other tube pedals, it is acting like a tube preamp but with lower heater voltages? Could I possible modify a tube pre, such as the High Octain preamp circuit found at ax84.com? If that is possible, then I can put in a dpdt (?) switch to bypass the second stage for clean tone with a slight distortion with heavy picking.

I'm not trying to make a SS amp sound like a tube amp. I basically just want toe nice saturated distortion, and a nice clean saturation. Any ideas would be helpful! Yes, I'm a noob to this, so thanks in advance for patience.