Hey Jay, the Guitar Mate Reverb III is also known as the YGM3, the third input jack is just an extra jack in the same circuit as the other two, your 69" might have been the last year for the 3rd jack, I've seen schematics of both 2 and 3 input jack models and it's the same amp. Have you seen how the original combo's were made? They had a divider that the amp sat on and the speaker section was a closed back design, I thought it was pretty cool. The Jensen should be a good speaker, I've heard that their design and use of ALNICO magnents is supposed to sound better with lower powered amps than ceramic magnents, but I guess the beauty is in the ear of the beholder. Hang on to that Garnet Cab, even without the original speakers it is still worth some money, hang on to it for a few years, Good Luck.............. If you need more info on the amp or parts, etc,etc,... try http://www.ampage.org