Background:

There are two basic kinds of IC based distortion circuits on the market, depending on where you put the diodes:

There's not much more to it than the schematics in above link, so you can take your average distortion stompbox, swap a few resistors and capacitors around and you get a pedal with the frequency response and distortion you want. Better, add a little internal array of DIPswitches so you reprogram the characteristics of the pedal if your tastes change. Discrete transistor distortion boxes are a different story.

The Ibanez Tube Screamer (TS-808, TS-9 and TS-10) is a volume boosting preamp with a tone filter which cuts the lowest and highest frequencies. It uses a silicon diode clipper to produce some distortion. Its is intended to overdrive the input stage of a tube amplifier, and its artificial midrange boost does produce a "screaming" tone for guitar solos. When it came out, it was popular because it was so different from the fuzzbox devices of the time. Now there are dozens of similar products on the market, but the original TS-808 has mystical rarity value.

It comes 1st in the Ampage "Best Distortion Pedal" survey, and 2nd in the Ampage "Worst Distortion Pedal" survey. I would be inclined to agree. It was the best of times, it was the worst of times...

The TS-10 is a product-improved version of the earlier TS-808, with some extra components to protect the internal circuitry. Some of the extra components make the pedal sound worse, but they can be removed, converting the low-budget TS-10 into the exotic TS-808 (almost).

Why is a used TS-10 cheap?

1. Most TS-10's have a more modern IC which does not distort as nicely as the original JRC4558D chip in the TS-808. The poorer distortion sound is what makes the pedal cheap. The "bad" IC can be replaced with a better one at little cost. My TS-10 has a factory original JRC4558D, as do many TS-9's.

2. The TS-10 has an extra transistor in the signal path when the effect is "off". This ruins your guitar tone when the effect is "off". You can remove two resistors and add a jumper wire to greatly reduce this "tonesucking" effect. This modification does cause the unit to make a short squawk if you turn it on with all the knobs near maximum. So don't turn all the knobs up to max.

The good news is that replacing the IC (if it is a bad one), removing 4 resistors and adding 3 jumper leads converts the TS-10 to a TS-9 standard. Changing 2 resistors converts the TS-9 to a TS-808. About half the changes have no audible effect, and are therefore optional.