My thought was to snip the ribbon cable and soilder to the ends of the jumpers.
That should work fine.
i don't understand why there is a jumper from hpf/lpf board that goes back to the pre amp board
i don't think that jumper is needed either.
Yes, it is. That's how the signal gets passed to/from the EQ stages and how the preamp board gets power.
Basically, the way the device looks is this:
Main amp
board
Preamp Board EQ1 EQ2 EQ3 EQ4 EQ5
Note that the preamp board is officially called board #1, EQ1 is board #2, EQ2 is board #3, etc.
You have the main amplifier board up above the row of six cards. The leftmost board is the preamp gain stage. Its output goes two places, depending on the position of the EQ switch:
A. to the main amp board.
B. to EQ5. Actually, it goes out of the preamp stage through what is IIRC the top pin on that jumper card (as seen from above and in front of the unit), and into EQ1. From there, it passes unaltered via a straight trace across that EQ board to the next jumper, to the next EQ board, etc., until the preamp's unfiltered output reaches the input of EQ5.
After EQ5 processes the signal, it sends the signal out via what is IIRC the bottom pin on the jumper card. The signal then passes across to EQ4 for further processing, then to EQ3, then to EQ2, then to EQ1, and is finally returned to the preamp card. From there, the EQ switch decides which signal to send to the preamp stage---the output of EQ1 or the unfiltered output of the preamp stage.
You're probably wondering why the main amp board also has a cable across to EQ5. This drives the clip light for the main gain stage and is not part of the audio path (the slight potential for noise injection notwithstanding).
Also, the +/-12V rails and the +24V rail are connected to the preamp stage exclusively via that jumper card. Without those rails, the preamp stage would be completely dead. The power cable from the power supply to the preamp board provides +48VDC for the phantom power, and that's it. Only the main amp board is powered by the other +24V rail.