The Wondermega also has the following DSP effect buttons (and level controls under the sliding control panel):

Pretty sure the X-Eye and Wondermega 2 don't have these.
True, I never use my Wondermega for Karaoke, but the DSP effects are far from useless! The "game" setting sounds absolutely amazing, especially on a good stereo system or powered headphones. If you like Genesis/Mega Drive music, imagine it with a lot more punch, and with crisper sound. For a game music fan the DSP effects on the Wondermega is almost as important to me as the S-video.