can subwoofers make things worse?

I've already accepted that I have to budget for 2 subwoofers of high articulation. But beyond 2 subs, seems the improvements is not worth the additional cost.  

How much improvement can one expect for each additional sub added?

The cheap power supply in the sub may contaminate your grid.

@soix 

You can make things better with the crawl method but you can not dial the subs in perfectly. The computer takes all of several minutes to make the measurement and generate filters that match the subs to the mains perfectly. No need to crawl around. Another benefit is you can put the subs where they function best, in corners and against walls and dial them in perfectly. If you have a sub against the front wall, but keep your mains 5 feet in front you have a delay of almost 5 milliseconds. The effect is to call attention to the subwoofers and they do not disappear.  This and the shallow slopes of the crossovers forces people to use very low crossover points negating many of the benefits of using subwoofers. In most situations the best results could be achieved with crossover points between 80 and 120 Hz. You have to use a very steep slope to keep the sub out of the midrange which is poison. This can best be done digitally. The high crossover point allows you to EQ the bass just by adjusting the volume of the subs. Boosting bass below 100 Hz by around 6 dB gives a more realistic sound and the visceral effect of music come through clearly at less than ear shattering levels.

@kennyc 

The only limit is the room available. As you raise driver surface area you lower distortion. This can be done with larger drivers to a point or multiple drivers. I use eight 12" drivers in four enclosures spread out across the front wall forming a linear array. My main speakers are also line sources. In short, the minimum is 2 subs. The maximum is what you have room for. I do not think there is a point of diminishing returns. 

atmasphere

The way room correction works is its an active EQ device that compares the incoming signal to what it hears happening in the room. This might be done with a test signal.

So at that point if there is a dip, the room correction simply boosts the signal at the frequency of the dip. The problem is that if the dip is caused by cancellation, you can put any amount of power into that cancellation and it will be cancelled. So the dip remains.

That is why you have to break up the standing waves before room correction can really work.

If your listening chair is located in a null, room correction isn’t the solution for better bass. Relocating the sub is. Multiple subs provide an even bass response distributed around the room.

_ _ _ _

A 50 hz bass wave is 22 feet long. A 30 hz bass wave is 37 feet long. These waves are bouncing off the walls, ceiling & floor. Integration of subs with main speakers in a small room can be difficult. But it’s well worth the effort if done right. There’s a lot LF in music. Why not hear what the composers intended?