Now it's subs

B&W DB1. You'll never listen without one after trying it out; and I mean in an excellent high end two channel system. Has auto room eq and is tight. Exudes quality as well. FWIW

+1 JL is good. Be wary of anything else - the vast majority of subwoofers are not suitable for music at all. It is all entirely about the quality of the woofer in the sub (JL make excellent woofers with long linear excursions) and EQ is just icing on the cake. Most subwoofers are just a bunch of distortion and that is most of what you hear - distortion. A good sub is actually barely noticeable with music but can suddenly be impactful. 

I bought 2 used RELs from the same era for 200 bucks each, both in excellent condition. The first was maybe 4 years or so ago and it's a Q150e front firing 10, and the second I found last year and it's Q108EII. My system was absolutely energized by the first and made even better by the second. Musical subs simply require that you move 'em around until YOU like the sound, and pay attention to the crossover points and levels on the subs (I find using a test CD to determine the ACTUAL low frequency limits of my mains is useful, and set the subs to take over around the point the mains stop delivering useful bass). I personally don't care about room EQ gizmos as you wind up surrendering EQ choices to the people who design these digital nannies, and I'm a seriously experienced live sound engineer who's ego won't allow such intrusions. Plus my largish listening room has tall, sloped ceilings and furniture. I turn the subs up and down if needed, and seriously enjoy the living bejeezus out of them every day.

If adding a sub to a 2-ch audiophile system - i.e., not a home theater receiver withy built-in gizmos - don't you need an external crossover, to prevent your main speakers from getting the bass signal? 

" If adding a sub to a 2-ch audiophile system - i.e., not a home theater receiver withy built-in gizmos - don't you need an external crossover, to prevent your main speakers from getting the bass signal?"

It depends. In many cases you don't need, or want to remove the bass from the signal with an xover. You adjust the sub so that it fills in where your main speakers naturally roll off. 

Another option is to adjust bass frequencies with software if you have a computer audio system.