Your sub experience: Easy or hard?

Like most anything it’s not hard if you apply the right tools in this case a sound meter or sound meter app.   

It wasn't hard at all, it was fun. I enjoyed the "subwoofer crawl",messing with the crossovers, plugging and unplugging ports.I have two HSUs and one SVS.If the HSUs ever stop working I would have all SVS.Controlling all of settings from the listening chair is wonderful.

I have two REL S/812.  The reason for two is mono block amps.  I found the setup to be quite easy and once dialied in, the improved sound is well worth it.  The only issue I have is playing the occational disk/LP that is very heavy in the low-end bass.  Then the subs are just a little too present.  But overall, they are spot on the majority of the time, and if I have to turn them down a click or two at times it no big deal.  

I have a pair of Sopra 3s that I tried to integrate with a pair of REL s/812s. I tried for months to get them to sound acceptable in my room, even broke out the REW software. But no matter what I did those RELs wouldn't integrate well enough.

Someone proposed the idea that I didn't have enough control over the RELs and that they wouldn't necessarily go deep enough anyway. They said try to audition a pair of Rythmik f12s instead because of the number of controls on their back plate.

Well after a few months I did manage to get a pair and I found them extremely easy to integrate into my stereo system. And it only took me a few days in total to get it right. I then gave back the Rythmik f12s, sold off my REL S812s and bought a pair of Rythmik G22's. 

Couldn't be happier... Although I'm thinking about buying one more pair of them.

It depends what you are trying to do / solve with subs.  I have an extremely difficult problem so the solution, including using 2 subs was also difficult. The problem could not be solved without subs as placement was critical, simply buying a pair of speakers or amp with more bass only made the problem worse.