"JAMMA+" is not a standard in any way, shape, or form. It simply means that there's a JAMMA harness "plus" some other stuff. You could start with a pre-made JAMMA harness, yes, then add the "plus" part, but the additional wires for the other 2 players involves almost as much wiring as the base JAMMA harness, plus you need all the connections to/from the I-40 board. Again, see the wiring diagram in the manual for Blitz/Showtime for more info. It's pretty detailed.
Basically, players 1-2 buttons hook up to JAMMA along with all the other "system" level stuff like video, power, coin door, etc., then there are some additional headers for player 3-4 buttons. The 49-way sticks are time multiplexed by the I-40 board onto IIRC player 4 joystick inputs, and the I-40 is controlled by yet another header ("AUX Latched Outputs"). The I-40 and 49-way sticks also need power, which is not a typical requirement for a control panel.
On a 2 player CP w/ 49-way sticks, the I-40 is not used, and the 49-way sticks for players 1-2 hook up to the player 3-4 inputs using both button and joystick lines.
Midway also had dedicated volume up/down buttons that you may wish to install. These hook up to yet another auxiliary inputs header. Volume can also be adjusted from within test mode.
If you want stereo audio, then there's yet another header on the board for that as JAMMA is, by standard, only mono.