NAD 3140 preamp problem.

If the Speaker Eq. switch is sometimes dropping a channel, in all probability it is just dirty - as far as I recall, it's a similar construction to the switches discussed in this thread:

https://www.diyaudio.com/forums/solid-state/373606-sound-loss-channel-sony-ta-f808es-ta-f555esr.html#post6692604

Unless you've fully desoldered, dismantled, and strip cleaned the switch, the contacts will still be dirty; spraying deoxit / contact cleaner into the switch is simply not sufficient to get to the contacts.
 
#20,
from the pic you provided, enlarge it,
the (A) and (B) sign, they looked very small.

(B) ( B inside a circle )next to C570
(A) ( A inside a circle )next to C569

you drew an orange circle in the pic,
beneath it (B) , the jumper wire on the board,
above it (A), the jumper wire on the board,
the jumpers looked like white in color from the web photos.

the jumpers go to C571, C572, (not in your pic, its in the original schematic.)
( in the delay switching section, closed to Q525, Q526,)

#21 bypass the spk. eq.

now you can carry out your plan to bypass the speaker. eq.
 
#20
in the pic you provided,

1)
disconnect from the amp board, unsolder,
(A) wire at the joint of C569
(B) wire at the joint of C570

2)
then unsolder and lift up the leg of R557, R558 that connect to sw6,
( unsolder and disconnect one leg that connects to sw6 from the amp board )

3)
solder (A)wire to R557,
solder (B)wire to R558,

this is how to bypass the eq.
 
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