As I've already mentioned, I own REL T9i. Actually, two of them.
There are couple of reasons I bought them. First, I bought into their marketing "There are subwoofers and then there is REL" which is a good line, btw.
Then I liked their connection possibilities. You can connect them with stereo system alone, with HT system alone but you can connect with both systems independently and have the manual crossover for stereo system and AVR's crossover for the LFE channel and bass managament. SVS has those similar features, but not a lot of other subs do.
Then, I liked REL's industrial design more then SVS. That visual aspect is important for me and i not ashamed to admit it. Most audiophiles are lying to themselves and others when they say that they only care about the sound quality and not the esthetics.
Now, the only flaw I see with REL T9i is that it there are no balanced inputs, because I have a problem with ground loops. Those high level inputs can help in that department, so for that it is not bad to have it. I would like to see measurements that show that high level input is inferior to line level. I might do it when I find the time. I have them on LFE RCA now only.
As for their low end extension, I have them at 5% volume setting on their knob and -6db in DENON AVR4500H and they are plenty loud and they rumble the room. I don't need more low end. Or maybe I do, but I am unaware of it.
I did consider SVS and still do, if I am to change/upgrade for T9i, but SVS is also very expensive in my country. The cheapest SVS PB-1000 is little more then 600 euros. SVS prices keep rising for better models, and the first model to feature XLR input (which is the only reason if i would upgrade) is SVS-SB4000 which is 2000 euros. And that is the black ash version (not pretty finish). REL has their S/510 with XLR input and 2000 GBP price, but looks beautiful compared to SVS.
Other subs to consider with XLR inputs are Pro Audio subs and their prices are competitive. Dynaudio 9s comes to mind with nice features, balanced inputs and outputs, fair looking, claiming 22hz low end extension, around 900euros, but I have not found any measurements for it.
Finally, I know that SVS has some better measured performance then REL but I am not sure if it contibutes much more in the whole experience. I don't believe anymore in REL's marketing claims, but knowing more now about subs (still not enough and still learning more everyday, here on ASR), I am not sure I would choose SVS over REL... but i might change my mind.
I don't think you should buy new subs just to buy new ones. If your current subs are rocking your house and they are not even turned up at all, they should last years and will give good performance. It seems you barely have them turned on. 5% volume setting is very low, so why spend the money? Just enjoy what you have!