Paralleling the low end is a good start, as getting as close as you can between driver sensitivities will help.
My (non OB) active system has an 8 dB sensitivity difference between the tweeter and woofer. I need to pad down my 103 db/1w/1m tweeter by 8 dB to match my 95 dB/1w/1m woofer. While I implement my driver EQ digitally via convolved IR filter, I implement the crossover right before the amps in the analog domain via Pass XVR-1. Using my measurement rig, I was able to dial in the 8 dB delta exactly using the attenuators on the front of the Pass. Once I figured out the ratio, I then played with dialing them both up until the woofer attenuator maxed out, and then both down until the tweeter attenuator bottomed out. I have enough total gain in my system that I ended up setting them with the tweeter attenuator bottomed out, as this was the (slightly) lower noise configuration, and I have plenty of volume range on my autoformer pre-amp.
You could perform a similar experiment with the DSP (at low to moderate volumes), and then pad down the other drivers with a suitable resistor to minimize the gain differences needed in the digital domain. You want to make sure that at high volume, you don't run into digital clipping on the woofer if you increase its gain digitally. In general, I'm very hesitant to increase gain digitally.