Subwoofer Basics Help

2 subwoofers seems to be popular. At what price point does a single subwoofer out perform using 2 subwoofers?

Two subs is popular for a very good reason.  Tough to answer your question in general because there are many differences within each brand’s lineup, but just to use one example I’d take two SVS SB1000 Pros at $950 over one SB3000 at $1100 especially if it’s primarily for music.  The SB1000 doesn’t go as deep as the 3000, but it still gets down to a legit 20Hz (-3dB) and you get the benefits of the subs not working as hard, providing a more even bass response, and dealing better with room modes/issues.  Just one example FWIW.

Yes the record out is full volume. As others mentioned your best bet is sub with high level connections like the REL. 
 

You could use the volume control route out of the record out but you would be doing a lot of fiddling with it every time you changed the volume.

The issue I'm looking at resolving in my system is my speakers don't fully open up on the bottom end at low volume. My system has to be "loud" in my wife's opinion to fully open up. I'm ok with this listening level most of the time but would like same at lower volumes. I'm also curious what is missing in the bottom end of my Naim NBLs. Gotta figure out how to demo something, would buying a cheaper used subwoofer give me taste of the improvements? I don't have a dealers close to me.