Assume a 6.8 Ohms resistor is placed in midrange crossover at the input terminal side and a 2 Ohms resistor is placed at the same position in the tweeter crossover. The difference between them is 4.8 Ohms (6.8-2 Ohms). If the woofer is replaced with a higher sensitivity one, the mid and tweet will need to lower their attenuations. And if the tweeter is bypassed its 2 Ohms resistor, would it be successful by simply replaced the 6.8 Ohms resistor of the midrange by a 4.8 Ohms resistor? By doing this, the difference between tweeter's and mid’s resistors still remain 4.8 Ohms (4.8-0 Ohms). That means the relationship between these two resistors is linear. Is this assumption true?
No. What are the impedance of the drivers?
If both drivers are 8 ohm, I would try a 3.9 ohm resistor for the midrange to keep it matched to the tweeter.
If both drivers are 4 ohm, I would try a 3.3 ohm resistor for the midrange to keep it matched to the tweeter.
I would get a range of resistor values. Start with the value from above that applies to your impedances, and measure the response with REW (free software, but you use a USB microphone, e.g., a UMIK-1). Adjust as necessary.
Lastly, you probably will need to tweak the crossover for the woofer. It is unlikely the crossover you have will work well with a different woofer, especially if the impedance of the new woofer is different than your old woofer. Side note, do not put a resistor in series with your woofer; it will negatively affect the damping.
Yes, the woofers will definitely have their crossovers revised. But, I didn’t mention it because I’d like to focus on the mid’s and tweet’s attenuations first.
Given the tweeter and midrange are both 4 Ohms;
Case I
R2 = 6.8 Ohms
R3 = 2.0 Ohms
Case II
R2 = 4.8 Ohms
R3 = 0.0 Ohms (bypassed)
Do these two cases provide resemble on-axis responses? As both cases have different value between R2 and R3 equals to each other—at 4.8 Ohms, hence, the relationship is linear?
No. That is not correct.
Also, looking at the schematic, R2 is not at the input terminal side. It is after L2. You probably also will need to adjust the value of L2.
Ideally, you would measure the frequency responses and impedance response of each driver, and model the crossover, e.g., in VituixCAD.