can subwoofers make things worse?

@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. 

I have been using, and building subwoofers since 1978. Subwoofers, the way they are commonly set up, virtually always make things worse. 

To make subwoofers improve the overall performance of a system you have to use at least two 12" inch subwoofers. Projecting bass beyond 1 meter down to 18 Hz requires a lot of driver. Defeating room effects to some extent requires multiple subwoofers.  A two way crossover is mandatory. 1/2 the benefit of using subwoofers is unloading the bass from the main speakers which increases headroom and lowers distortion. Subwoofers have to be matched to the main speakers in phase and time. You can be in phase and out of time or in time and out of phase. Achieving this requires measurement and digital bass management. This is not just "room control" which is really speaker control. It is crossover choices, time and phase alignment and room control which is just amplitude adjustment. You also usually get EQ capability which allows you to dial up the volume in a controlled fashion.

@soix 

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. 

@epz 

I would forget the sub for now and spend the money elsewhere. It does not matter where your adjoining neighbor is, any effective subwoofer system will drive him nuts. I am lucky to be alive. How I ever made it to 30 is a mystery. 

Who remembers drive in movie theaters? You hung the speaker on your side window rolled it up and asked the age old question, "you're on the pill, right?" 

@sierratrails 

Yours is a classic story. The problem was the available analog crossovers were way to slow and there was really no way to phase and time align the subs. Digital electronics and digital signal processing were still decades away. I had electrostatic speakers and spent the better part of two decades trying to make subwoofers disappear. They always managed to creep into the midrange in stark contrast to the ESLs. I cross now at 100 Hz 48 dB/oct. The ESLs sing and the subs stay out of the midrange. Because the crossover is digital there are no ill effects. (other than being digital which seems to bother some people)