The REW alignment tool makes it easier to determine delay timing. It can be used to time multiple subs together as well as determine the sub group delay needed to match well to the main speakers. There are other ways to do both. MSO can find a very favorable timing for the individual subs, but I have no experience with it. Some of the automated methods work very well for sub group delay timing and there are many manual methods.
No matter the tool, first determine delay settings for the individual subs. The sub group delay to the mains can then be determined.
If the REW alignment tool is going to be used for either of these. I can probably help if needed. It's important to have the correct measurements however. Here is link to the
Needed measurements for determining the delay of the subgroup to the mains.