A nice upgrade for your KT-8300.  This was made by the well-known FM enthusiast Bill Ammons, who made several different boards for various tuners to enhance their performance.

More info is available here:  https://fmtunerinfo.com/AmmonsKT-8300.pdf

Description:

The KT-8300 Dual IF Band Stereo Separation Control PCB upgrades the KT-8300 to provide a second set of stereo separation controls. In stock form the KT-8300 only has one adjustment control per channel. Normally this control is optimized to give the best stereo separation in the WIDE IF mode. When the unit is operated in the NARROW IF mode, stereo separation is not optimized. As the NARROW IF bandwidth is decreased further, stereo separation drops quickly. Four 15 turn sealed pots allow easy drift free adjustment of stereo separation This new PCB can easily provide an additional 15 to 25 dB of stereo separation in the NARROW IF mode.

Shipping included, feel free to make a reasonable offer.