I added LED's to my KISS several years ago and it certainly brightens things up. Just like any old Bally game, you get flickering on all the underside interest. The top-side general illumination look great though. When I have time, I'm going to put incandecents back in all the inserts. The game isn't on more than a couple hours max if I have people over or when it's just me, I play a handful of games and turn it off.

LED bulbs make the backglass really pop and highly recommend put them there. Just avoid anything under the playfield unless you get that special board that illuminates the flicker.

I recently converted my Bally KISS over to full LED. I replaced the original lamp board with the Alltek Ultimate Lamp board to eliminate flicker. With LED's you can choose to keep the look of the original with warm white but I chose to brighten things up to look more modern. That is the beauty of LED's (besides the power savings and temperature reduction of course) is that you can custom design your look. I finally decided on warm white behind the backglass to keep that look original and went for brights everywhere else. I can really tell when sitting next to my Stern KISS. Love it!

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The Siegecraft adapters are easy to install, inexpensive, and work great.
http://www.siegecraft.us/presta/index.php?id_product=44&controller=product&id_lang=1

Edit : Crap, just noticed that shows out of stock. He may be changing styles of board.

I have a Bally KISS machine w/leds. I use the Alltek LDB. The Siegecraft adapters work well too. I did not have to do anything to the aux lamp driver board. The strobing logo works fine. I used Cointaker warm white bulbs and I'm happy with it.