Keeley TS-808 TubeScreamer Mod Ibanez Pedal (Mod Plus) -used  From Keeley

The legendary 808 with the finest, most popular set of tone mods you could dream of, The Keeley Plus Mod and True-Bypass clarity and fidelity.

The new TS808 offers a vintage look and sound. Many people are wondering if it needs mods since it's the Holy Grail TS808 with the new JRC chip installed. Sure it does! If you look at the board, it's the exact same as the TS9 and has the same "faults," if you will. Areas that need improvement are limited bass response that cuts so much that it appears as a huge midrange hump in the sound. There is also a limited gain range which means it doesn't get clean enough or on the other hand, doesn't get enough distortion. So to combat these issues we increased the gain range allowing it to get cleaner and more distorted through the range of the drive control. Also, the lack of transparency or muddiness becomes a problem for many users. All these things are fixed with our mods. Our approach is to give you the most out of this classic pedal.

Here's a little quote from the Ibanez Web site. It gives their perspective on why a better sounding chip can make a slight difference.

"However, to get the warm sound of the original TS808s, Ibanez can't use just any currently manufactured JRC4558. In over two decades since the original TS808's were made, the manufacturer has made improvements in consistency, electrical characteristics and fidelity of the JRC4558.

Unfortunately, those aren't necessarily 'improvements' when your goal is vintage distortion. (And don't let anyone tell you that a 'chip is a chip is a chip.' Chips vary from country to country, factory to factory and even foundry to foundry within a factory.)

The result is that present-day chips are different than those of the 'good old days.' However, we discovered that we can pick out a small quantity from current lots of JRC4558 chips that match our vintage TS808 spec, if we measure them one-by-one to find those with the desired characteristics. This careful and labor intensive selection process limits the production of the TS808 and increases its cost (as does its famous old square foot switch, which had to be completely recreated in order to make for an accurate reissue)."

SO, Ibanez has tried to pick out some good ones to make the pedal sound a bit better from the start.