How low do you let your subwoofer play? One way to _estimate_ how much power a subwoofer will handle is to model excursion in WinISD. I care about SQ and SPL. Therefore, I consider a subwoofer power handling limit is where it runs out of its xmax excursion. The can be lower or higher than the manufacturer's RMS rating. Let's say you run the sub sealed. Load the box specs in WinISD, switch to excursion model and start adding the wattage in signal section until the graph exceeds xmax excursion at about 25Hz. For ported subwoofers, the highest excrusion will happen at or above the tuning frequency, if you dial in the subsonic filter setting.
I just did this for SWR-1243. WinISD recommends 1.9 cu ft sealed box for best qtc, which is kind of large from utilitarian point of view. 1.2cu ft box results in 0.779 qtc, which is very reasonable. Looking at excursion graph, with 600watts, cone excursion is only 16mm at 20Hz. With 1000watts, excursion is at 20mm at 30Hz, and 21mm at 20Hz. So this subwoofer has enough "excursion headroom" to handle 1000watts. However, this modeling tells us nothing about thermal limit of this subwoofer. I would run 600watts to this subwoofer as recommended by manufacturer.