You might want to start by reading this. https://www.acousticsciences.com/wp-content/uploads/2024/11/Soundoctor-Barry-Ober-sub-integration.pdf
My guess is that your acoustic engineer was adding delay to your subs so they would be in phase at the listening position.
To my knowledge, all powered subs have some type of e.q. circuitry in their design which also creates a delay of a few to several milliseconds depending on the model, coupled with the delay caused by normally placing your subs further away from your listening position than your mains, you could start getting a significant delay between your subs and your mains, so I am surprised he didn’t say anything about delaying your mains since he was concerned with a 3 ms delay between your subs.
Your driverack is primarily a 3 in, 6 out active electronic crossover with DSP, that also gives you a 30 band,1/3 octave e.q. along with an 8 band parametric e.q. on each input. On each output you will have both low and high pass filters with selectable slopes. 8 band parametric e.q, driver protection and driver alignment delays. It is a pretty powerful piece. There is also software that you could download onto your laptop that would allow to to operate the driverack from your listening position. You would also use the driverack to set the volume balance between your subs and mains so one doesn’t overpower the other.
Your CR-1 gives you an active crossover with either a 2nd or 4th order Linkwitz-Riley slope. You can also control volume balance between subs and mains and both units will give channel muting capabilities.
If you do not use either unit you would want to set the low pass filter on your REL’s around 34 Hz, not 120.
I would not rule out using DSP on your mains until you have at least tried it. If done correctly the positives should be far greater than the negatives.
