I prefer two subs. But with subs, wave effects in the room are always a concern.
With one sub, I had to be careful with room nodes and resonances. Proper location of the sub would avoid nodes and resonances, at least the major ones. Also, I discovered when I had just one sub that the two inputs from the two channels were treated differently, with one of the two channels significantly deemphasized somewhat arbitrarily so as to avoid cancellations because the bass waves could have exceeded the distance between microphone positions. The good news is that the best placement for the sub was different from the best placement for the monitor speaker, and having them separate allowed me to find the best placement for each. If they were all part of one speaker box, placement could only be optimized for one.
With two subs, the potential cancellation effects have to be dealt with through room placement. At least, I can dedicate each of the subs to each channel and not worry about the built in deemphasis.
With one sub, I had to be careful with room nodes and resonances. Proper location of the sub would avoid nodes and resonances, at least the major ones. Also, I discovered when I had just one sub that the two inputs from the two channels were treated differently, with one of the two channels significantly deemphasized somewhat arbitrarily so as to avoid cancellations because the bass waves could have exceeded the distance between microphone positions. The good news is that the best placement for the sub was different from the best placement for the monitor speaker, and having them separate allowed me to find the best placement for each. If they were all part of one speaker box, placement could only be optimized for one.
With two subs, the potential cancellation effects have to be dealt with through room placement. At least, I can dedicate each of the subs to each channel and not worry about the built in deemphasis.