JL Audio Fathom

I have a pair of F112 v2 running in my system.  They are very well integrated and I've had no operational issues.  I'm very pleased with them.

In general, it's difficult to get really good sub integration.  Since you are happy with the integration in your system I wouldn't change a thing.  

Not worth it. Money is better spent on another part of your system like your phono or digital. Even better, room treatments. Way better to control your bass with room treatments. 

I wanted to come back to this after some thoughts.  One of the very important reasons JL subs sound so good is the quality of the built-in room correction.  I really like the choices it makes, but at the same time you can do the same with a microphone, Room EQ Wizard and miniDSP for a lot less.

If the OP is genuinely interested in going to the next level with his setup, I'd encourage him to go that route, which is going to be a lot less expensive but also more challenging.  The good news is he can't really break anything that way.

As I mentioned before though, bass traps when needed can really help, but that would come from measurements.

If you haven't already, perform the subwoofer crawl and map your rooms two loudest standing wave bass modes. Position the ML 1100X's in those modes then recalibrating them. Despite the level of noticeable improvement at least you'll know that rooms optimum sub locations, forever.

The cost of custom length subwoofer interconnects from say, Blue Jeans or Monoprice Cable are quite economical. Try and maintain the same VAC supply to avoid grounding issues.

A -6dB  'Sub-Bass' speaker is not a subwoofer regardless of where its manufactured.  

I have 3 F112 v1, since 2008 iirc, I’ve also had need to have them serviced, keeping my boxes shipping them to either AZ or FL. The service is excellent and timely: about 7-10 turn around. The fact that they can be serviced is a plus. That they are still in business even with some negative feelings by a few says a lot about them as a company. The good news is they are stunning, powerful, fast and adjustable in every manner. I don’t use the ARO room correction but let my Trinnov deal with it. My system is a dual purpose room, both analog and HT. I play vinyl and digital music plus movies. Getting 3 subs in phase with the mains is challenging, I’ve done it manually in the past, but not much need now. I don’t think you’d go wrong getting the F112.