increase power amp sensitivity

Phono Input is probably set-up for MM cartridge and you can change resistor value. The sensitivity of the phono input won't change.

I believe that the OP's reference to "phono input" was referring to the connector type (RCA), not to the signal type, and that a phono level signal is not involved. I suspect that reflects British terminology.

Negative feedback resistor if lowered will bump the input sensitivity, but your amp may go unstable and scream at idle.

Given that an op amp stage is being addressed, and assuming that it is not set up as a unity gain buffer, the gain of the stage will be raised if the value of the negative feedback resistor in that stage is raised, not lowered.

Good suggestion by Heyraz.

Regards,
-- Al

If the attenuator plugs work out for you, once you roughly figure out how much attenuation you need, you could put a "passive preamp" (stepped attenuator switch) before the louder amp to fine tune the levels.

As Heyra has said, its going to be a lot easier to reduce the input to the bass amp than trying to increase the gain for the high frequency amp!

Phono Input is probably set-up for MM cartridge and you can change resistor value. The sensitivity of the phono input won't change.
Same may apply to line stage resistors.
Negative feedback resistor if lowered will bump the input sensitivity, but your amp may go unstable and scream at idle.

Like Al said, there's not enough information in your post to make an informed suggestion. But I had another idea that may be a worthwhile alternative.

Why not simply attenuate the (louder) woofer amp before modifying the tweeter amp? Parts express sells attenuating plugs, I would try that before changing an amp's input. It's a cheap and reversible.

I assume from your question that the active crossover does not provide a means of independently adjusting the amplitudes of its high frequency and low frequency outputs.

I'm a little confused, though, by the reference to the 100 ohm output impedance of the preamp. Is the active crossover function provided within the preamp, or is the active crossover a separate component that is connected between the preamp and the power amps? In the latter case, of course, the output impedance of the crossover (not the preamp) would be what is relevant with respect to the input impedance of the power amp.

Re question 1: 200K is undoubtedly already so high in relation to the output impedance of what is driving that input that removing it will have essentially no effect on gain.

Re questions 2 and 3: Can you describe the circuit more fully? Am I correct in understanding that the 20K resistor and the 500 nF capacitor are in series with each other and with one of the inputs of the op amp (as opposed to one end of the resistor being connected to ground)? If so, would that be the inverting (-) or non-inverting (+) input of the op amp? And what other resistors are connected to each of the inputs of the op amp, and what is the value of the resistor in its feedback loop?

Finally, although there are some exceptions, very few line-level output stages in consumer audio equipment can drive 100 ohms (or anything close to being that low) with good results. Although depending on the answers to my questions about the circuit configuration, changing the 20K resistor to 100 ohms may result in an input impedance that is very different than 100 ohms.

Regards,
-- Al