Your experience of moving to two subs

I've just started playing with this in a PC system.  I'm buying small subs off ebay for cheap and now have 4.  2 tiny PSB sub series 100s and 2 Episode 8" Evo subs.  Episode is a custom install brand and has terrible resale value because nobody knows what they are.  I took a small chance for about $150 including shipping and it's definitely better than I'd expect for that price.  They're not blow the doors off HT subs but get you down to the high 30s.  All 4 subs are sealed.  

@curtdr, Honestly subs are more for movies or just messing with.  They do make a difference with some music but if you mostly listen to music that doesn't have much below 40 hz you don't need them.  If you want good bass the most important thing is to live in a lightly built wood frame house that lets a lot of the bass escape through the walls.  

One thing that could lead to being able to locate subs could be that they're causing nearby objects to resonate at higher frequencies and you're hearing that.  I'd play around with tone generator a little to see if you can isolate anything.  

Long ago in my obsessive phase I made an extremely dead sounding room in my basement.  I made dozens of insulation bass traps, got equalizers and measurement mics and found out what accurate was, at least from a frequency response perspective.  One thing I realized is that if I clapped near one of my components it made a very audible resonance sound.  If something coming out of the speakers set that off it would have been a lot more added distortion than the component put out.