I was not going to intervene, but I was encouraged…
I can enjoy several systems, 3 of them in room with integrated subwoofer, which brings several advantages: full range system for a much more reasonable budget than using only floorstanders, large opening of the soundstage and better focus (both in quantity and quality).
IMHO the key lies in the quality of the subwoofer and its integration; but if the sub is not of great quality, and preferably sealed if it is to listen to music, or preferably to music, there is no integration that is worth.
After trying several subwoofers, I finally opted for SVS (SB16 ULTRA in the main system and SB2000 in the living) and Jl Audio (D108 in bedroom), which are among the best in subwoofers, although I would also consider Rel and Velodyne.
It is not so complicated, and it is being tuning, like everything:
The bass level: halfway through the level, to finish adjusting it at the end, usually increasing it slightly.
The crossover frequency: start with a frequency between 5-10 Hz above the minimum in the room (-3 dB) indicated by the speakers manufacturer for them, and adjust according to preferences.
Phase: 0º if the subwoofer is firing in the same plane or direction as the speakers, and 180º if they are firing in different planes (for example the sub behind the listening position and pointing forward); anyway, it is easy to try until you discover the one that convinces the most, almost always quite invaluable given the omnidirectional sound dynamics of the bass.
Of the group delay I would not worry given the dimensions of most rooms, the speed of sound and reflections of different order.
This reflections are very important to prevent the sub from exacerbating the room mode resonances; but the only control variable is to play with the positioning until finding the right point of integration. For example, in my main system, I calibrated de big SB16 to sound extraordinary in a sweet spot of around 1.5 square meters, which is where I usually enjoy it, and allows almost 3 comfortable people to take advantage of the sweet spot; in the rest of the room, which is large, there are points where the bass is more intense and another where it is much more subtle, but without awakening the room modes at any point.
A subwoofer is known to be well integrated when you don’t notice that it is or it is acting; but if you disconnect it, the soundstanding and focus are drastically reduced.