Do you have any way to put a steady signal into the Cobra's receiver?
I don't think your direct-inject mod is likely to be part of the problem. The hookup for C174 is not anywhere near parts of the receiver that could reduce its sensitivity.
Unless maybe you, ah, "splashed" a glob of solder onto the pc board in the receiver section.
But when the sensitivity of a receiver drops all at once, the first thing I ask is "was the antenna connected during a storm?"
Lightning can strike a neighbor's tree, or a utility pole down the road and still cause a destructive surge in your antenna.
The reason for that steady signal source, one you can hear in the 2000 is to see what happens to the alignment adjustments.
Any of them that do not show a peak point to a problem in that section. If the slug-tuned coil that's the first stage where the antenna feeds into the receiver shows no peak at all, this suggests a surge came down the coax.
Another common failure is seen in the 'cans' with the small-diameter tuning slug. When the capacitor inside the can goes bad, the slug will now appear to peak with the top surface of the slug dead-flush with the rim of the opening in the top of the can. When you see this, it's not really a peak. It's just the end of travel for that slug. Continuing to turn it counterclockwise will cause it to extend above the rim of the hole. This will have the effect of 'reducing' the coil's inductance. Same as turning it clockwise, so that it recedes inside the can. This "dead flush" position of the peak is not really a peak in the normal sense. A true resonant peak will always be one turn or more below the rim of the opening.
But yeah, attempting alignment is a diagnostic tool. That's what I would do next.
73