The Akai CR80 series did have a button next to the 2CH/4CH display, but it only allowed carts without a notch (2CH mode) to switch to quad (4CH mode). If the cart has a quad notch, only the quad mode can be used (4CH mode), and the button is disabled. I'll bet there was some interesting meetings in the Akai Marketing Department on that decision. The notch is also used in features on other models. The play-only Panasonic QH-666 unit will sense a stereo tape and duplicate the stereo signal to the built-in rear amplifier keeping all four channels active with double stereo. The Akai and Technics decks just shut off the rear channels in stereo tape mode.