can subwoofers make things worse?

They certainly can make things worse! They can also make it much better if it’s dialed in correctly and time aligned with the mains. Also the type of bass. If you have a tight fast sub with a lazy bass speaker it will probably not blend well and vice versa. 

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)

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.