Not knowing the power and capabilities of your sub, I will say that placing the sub in the corner is giving you some sonic challenges. However, it is boosting the bass output of the sub in the room. So, there's tradeoffs. You would get better quality bass if you put the sub where it is getting "flatter" response. This often means out in the room more, maybe nearer the front speakers. However, you may then have to try to overdrive the sub to make up for the lack of bass output.
A solution would, yes, be to add more sub(s), to add efficiency. Then, you simply will have to spend time finding good locations for both subs, and get phase right, etc.
If you find a good spot for one sub, where you're getting reasonablly flat bass response, you can then put another sub elsewhere in the room possibly. I would ONLY DO THAT, if I needed to counter a bass mode problem where the first sub is at. If, say, there's a peak from the first sub at a frequency, I'd try to place the second sub were it was in "the null" at that frequency, and vice versa. This would help smooth out bass mode problems, and provide smoother bass. HOWEVER, if you find a place where you're getting very flat response from the first sub, then I'd try placing the other sub right next to or on top of the second! That makes things easier, and you'll note have to worry about comb filtering phase problems, because all effected frequencies are longer than the distance between drivers.
In your case, sounds like, for best overall sound, you need, yes, another well placed sub possibly.
Still, spend time getting best sound from the first sub, then add another and experiment
A solution would, yes, be to add more sub(s), to add efficiency. Then, you simply will have to spend time finding good locations for both subs, and get phase right, etc.
If you find a good spot for one sub, where you're getting reasonablly flat bass response, you can then put another sub elsewhere in the room possibly. I would ONLY DO THAT, if I needed to counter a bass mode problem where the first sub is at. If, say, there's a peak from the first sub at a frequency, I'd try to place the second sub were it was in "the null" at that frequency, and vice versa. This would help smooth out bass mode problems, and provide smoother bass. HOWEVER, if you find a place where you're getting very flat response from the first sub, then I'd try placing the other sub right next to or on top of the second! That makes things easier, and you'll note have to worry about comb filtering phase problems, because all effected frequencies are longer than the distance between drivers.
In your case, sounds like, for best overall sound, you need, yes, another well placed sub possibly.
Still, spend time getting best sound from the first sub, then add another and experiment