The hard part with gain staging powered equipment from different manufacturers is that the input clip point and the input sensitivity of the amp may be different. Turning one unit down so you can drive another one harder does exactly that, but does nothing for your frequency response and acoustic performance.
If you need/want more sub activity, the acoustic crossover point will shift with it and without being able to actually measure and adjust those settings, the activity of doing so is literally a matter of taste at that point. So if you want more low end and to still be able to gain stage the whole system, you either need more subs, or you turn down the tops. Then we address input sensitivity. If one unit has a 26db fixed gain input sensitivity and another unit has a 1.14v input sensitivity, they will acquire acoustic output slightly differently from one another. One will get louder sooner and clip sooner than another as well. This may completely change how the system as a whole is staged. Most powered units simply have a max input level that is typically around +20dbu. Now if you have similar input sensitivities but different peak input capabilities, you cannot really gain stage per se and adjust the systems acoustic balance as well. Since all the units will be running equally, the difference in peak output is what will determine the base frequency response. If the tops produce 136db and the subs only 130db when clipped, you will always have a deficiency of -6db from the subs. The only way to change the balance is to turn down the tops. Keep in mind however that the inputs of the respective speakers will still clip at the same relative input level, so turning a speaker down simply means you have to drive it that much more in order to clip it. You, in essence, can change the relative outputs to make it so that they all clip at the same time, but as mentioned, you lose the ability to balance the system.
One way around this is by going with subs on an aux. Since you can gain stage the system, you know that if you see clipping here, it's also clipping there right. Well with subs on an aux you can create balance in the mix directly. Need or want more or less bottom end? Simply turn the sub aux up or down on that given channel. The downside with this approach is that true system linearity is sacrificed in order to create the desired balance. The only way to have system linearity is to have the subs and mains tuned so that at X drive level, the system is performing as desired. If you alter the drive level to either half of the system, you will have a deficiency or an abundance of output from one part or the other. This is not usually a problem but is something to consider. This is why we say simply turning the subs up or down to the desired balance is the easy way to go. If at the very least, the system balance is good, then most of the other shortcomings are less of an issue or at least easier to sort out.
With powered units gain staging is really a moot task. If you balance the system to a desired frequency response, you will almost invariably be turning the tops down to create the desired balance. This means the subs will always clip first. At what level they clip at is typically easier to determine than sending level to them and looking at them. Most all powered units will say what the max input level is. For the RCF 8004 that level is adjustable between -2dbu or +4dbu as best as I can tell. So if you turn the sensitivity all the way up to +4dbu, you already know that when you hit +4dbu on your output meters, the subs will then be clipping. Most mixers are capable of producing around +18dbu to as much as +26dbu; so capable of much more drive level than is required to clip most any amplifier input.
The DRPA is nothing more than a digital tool that could be used to deal with that issue, but so is turning the speaker up or down. I don't suggest using the DRPA as a crossover for the subs. The subs have a selectable crossover and it is non-defeatable, so if you use the crossover in the DRPA for the subs, you would be stacking crossovers on top of one another. This can create several ms of phase shift and create a very unique and interesting slope to that phase shift. Keep it simple. Use the shortest path you can. The tops often don't have a crossover built in and using the DRPA for that purpose then makes sense.
