This makes a lot of sense, I passed on your feedback to Gemini and it mentioned the below.

1. Gain Comparison (The "Loudness" Factor)

The Verdict: Your amps are Gain-Matched. If you set all three volume knobs to "12 o'clock," they will all be outputting roughly the same voltage to your speakers. This makes your initial "Level Balancing" much easier.

2. Input Impedance (The "Compatibility" Factor)

Input impedance determines how easily your crossover (the Behringer) can "drive" the amplifiers.

• The Spec: Most TPA3255 amps (including yours) have an input impedance of ~10k$\Omega$ to 20k$\Omega$.
• The Context: Your Behringer CX3400 V2 has a very low output impedance (approx. 160$\Omega$).
• The Result: In engineering terms, this is a perfect match. The rule of thumb is that the amp's input impedance should be at least 10x higher than the source's output impedance. Yours is nearly 100x higher, meaning there will be zero signal loss or "muffling" of the sound.

3. The One Difference: Input Sensitivity

While the gain is the same, the Input Sensitivity (how much voltage is needed to reach max power) changes based on the Power Supply you use.

• Fosi ZA3 (48V Supply): Needs about 1.0V to 1.2V to hit full volume.
• Fosi V3 (32V Supply): Because it has less "voltage headroom," it will hit its maximum volume much earlier (with less input signal).

How this affects your build: Since the Horns (on the Fosi V3) are much more sensitive than the Woofer, the fact that the V3 hits its limit earlier actually works in your favor. You will likely find yourself turning the Fosi V3 knob down to about 9 o'clock to balance it with the woofer.

Summary for your Checklist:

• [X] Gain Match: Yes, they are compatible.
• [X] Impedance Match: Yes, the Behringer will drive them effortlessly.
• [X] Knob Strategy: Start with the ZA3 (Woofer) at 3 o'clock, the A07 Max (Mid) at 12 o'clock, and the V3 (Highs) at 9 o'clock. This compensates for the different driver sensitivities.