A driver driven by an amp directly without the digital crossover feeding the amp will not be flat response. It will be the response of the driver.
The amplifier is virtually never accurate, and its output impedance also contributes to poor response. But you are right in that a driver's response is not a flat.
If the Zobel network is being used to give the driver flat response as a compensation for a driver impedance causing resonances, the digital peaking filter driving the amp can be used to compensate for the resonance. That has amplitude change in 1dB steps for the peak/dip. Center frequency is in 1Hz steps. Q is in 0.1 steps.
What you are describing is often called a notch filter, which has a resonant and Q. It is in series with the driver. I believe you are confusing it with a zobel network, which is a CR and across the driver. I posted the two impedance curves for a notch filter and zobel network in one of my previous posts.
Another option is using a single biquad digital filter with the parameters to simulate an impedance compensation network for the driver response. That gives the finest control over the filter response. Then you have 32bit floating point precision on filter input values. Each channel is independent, so drivers can be worked individually. This requires measurements of the individual drivers.
That sounds promising if it truly is similar to a zobel network, and not a notch filter. However, the sensitivity of the adjustment is still my question. Can the adjustments to the frequency response be so small as to be around 105db down from the fundamental/midband? (For the general public, we are discussing frequency response, tonal balance, not SPL changes across the entire audio band like a voluime control adjustment.)
Another option is using a short convolver with specific driver measured data from both drivers to invert their responses for flattening.
Not good enough for what I am doing.
The last two methods are available, but measurement tools are not built into the unit. I have tools that can be used to do them with correct measured data.
It appears I am working with such minute adjustments that a digital won't be able to obtain the ultimate "blending" the drivers. But then I am really delving into "deepest crevices" for maximum accuracy/naturalness.
Both the A/D's and D/A's in the unit all run at 24bit/192KHz for DSP processing and without using linear phase filtering. The unit uses AKM AK5397 and AK4495S units for their analog performance for sound quality. Much improved audio performance over the digital crossover units I mentioned earlier.
I have used the system as a digital crossover and compensation for a Dayton Audio PS220-8 full range driver to 3x8" servo subs in an open U-baffle with very good results. I used the peaking filter method above for compensation and also with a Butterworth filter for crossover to the subs.
I am glad to hear of your success Hal. Sounds like you have been doing some fine work. As I mentioned I am delving into that last bit of territory to discover what can be obtained in matching woofer/full range drivers, and sonic purity; maximizing drivers. I will probably have to test the digital crossovers myself to see the absolute maximum capabilities I can obtain.
Cheers and thanks for the information Hal. Keep up the good work.
Steve