Keys 7 and 8 from the bass side are lower than

Sounds like either there is a problem with a low frequency driver or perhaps an eq setting.
If you have an external (line) output, you can try sending the signl through another amp. If you have headphones, then try them also. If the headphones sound distorted as well,then see if you have the eq to high on the bass.
Other than that, it could be a filter capacitor in the power supply. You would need some experience fixing that.

The low in side jack has a lower vol. but should only be1/4 less than the high in jack on most amps, --try this,-- plug guitar in high in then take all the treble off and 1/2 mid. range bring up bass range and if you have equauzation bring down 26k 5k 10k about 2 notches or points. See if that give you better bass tone.

To remove the knob, there should be a small hex key or allen screw on the side of the knob. Just loosen the screw. You don't have to remove it.

To tighten the nut properly, you may have to remove the back panel of the bass and hold the potentiometer from the other side. If allowed to turn too much, it could mess up the wiring.

Verify the sound in headphones directly from the unit. The cymbal MAY be panned to one side or the other and will require higher frequency response than some of the piano notes.

A SIMPLE way of looking at the transposer is that it shifts EVERY key on the keyboard left (+) or right (-) by the number of the transpose. ALL keys, black and white are counted. As far as counting sharps and flats I would NOT depend on that. The transpose would for instance allow you to play a song you knew in "C" and have the sounds moved to a different key. For instance, a transpose of +2 would move EVERY C to "sounding" D, C# to D#, D# to F, E to F#, F to G, F# to G#, G to A, G# to A#, A to B, A# to C, and finally B would sound C# pitch. Given a key like G for a song that has one sharp (F#) to play this by keying it in the key of "C" you would use a transpose of +6 or minus 6. The number of key positions (including black) between the key you want to play in and the key the song is in. For a song in D that is played in C you would use either a +2 transpose or a -10 depending on whether you wanted the higher or lower octave to sound. Looking at it this way I do NOT believe the method you proposed would work... Look at it the way I described and you won't go wrong.

The wire from pin 1 or 7 of CN1 on the GHL88L (left end) circuit board is open where it goes to the middle circuit board. This could be a bad cable, improperly seated cable connector, or a crack in the circuit board(s) connecting to these wires. These are the common matrix wires associated with only the C# and G keys for the left 1/3 of the keys. If more than 1/3 is dead, then the problem is likely on the middle board GHL88M that connects to the mating cable to the CN1 connector.

There are two wires dedicated to the keys for the two contacts that are used to measure velocity of the key strokes. There is a mechanical position between closing the contacts and the time is measured by the processor as velocity.

If you are the least bit worried about working on electronics you best have a service person do the repair. The keyboard has to be lifted out after removing the top and disconnecting other cables.

Check that the front of the keys is not hitting the lower plastic case front. If they hit it, then you need to disassemble the case partly and see what has shifted. It is ALSO possible that something has caaaused the aftertouch sensor to move out of position and it is hitting the keys early. ANY of these conditions needs to be addressed before damage is done.

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