SVS makes really good subs and they have great customer service and they are reliable. I have 3 of them, one SB-12 NSD in a bedroom system and two SB-13 Ultras in my main system, combined with two Rythmik F25 subs.
I have heard Rel subs and think they are great, but I have a hard time justifying the price difference between Rel and many other brands. There isn't enough sound "difference" IMHO to pay twice as much or more for the same level of output/performance.
I'm scratching my head a bit about the comment about them being too loud. The two in my main system have the volume set at something like -14 and -15. When I bought my Rythmik subs, I bought them pre-owned. The owner said something like "I have them set neutral" which in his mind meant that he had his AVR set at 0, but I don't think they were in any way calibrated or "neutral". They were overwhelming. You probably could have heard them half a block away the way he had them set up. In my setup, the volume is at about 9 o'clock.
Having them set up so that the volume control can be set low is a good thing. That means that when called on, they will respond with ease and have lots of overhead and when they are not a prominent part of the musical content you may not even realize they are there, when properly integrated.
Having lots of overhead is generally not considered a bad thing when discussing subwoofers. Some subs can be too big for the space, which is why there are multiple models in every sub manufacturer's lineup.
If a sub isn't good for music, it's not going to be good for home theater either and vice versa. I do however prefer to stick to sealed models for music.
Before you purchase either a Rel or SVS, I would suggest looking at Rythmik. I like their servo technology and would recommend them over SVS and you'll get more value for your dollar than you will with Rel.