can subwoofers make things worse?

The phase adjustment on subwoofers is very close to totally worthless. Trying to adjust subwoofers by ear is extremely difficult it not totally impossible. Measurement is a must and unless you are extremely lucky the only way to dial in a subwoofer array so that it disappears is with digital signal processing, otherwise you have a home theater system with which fidelity takes a back seat to vibratory excitement. 

@mijostyn This is not my experience at all.  If you take the time to use the “crawl method” to find the best placements for subs (critical) and then take a little time to appropriately dial in the crossover, volume, and phase you can get subs to totally disappear in a system and provide all the sub magic.  I don’t think it’s luck, I think it’s having some knowledge and the willingness to do the work to dial it in.  That’s my experience anyway.  Agree at least two subs are necessary and placed in a staggered arrangement to deal with room modes.

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.