I have a few thoughts as a sub user for 20+ years now. I have heard the brands you are discussing and many others. I am firmly in the camp that all systems improve with prosper implementation of subs.
short answer, will the SVS sound as good as the Rel and cost less?… yep… and a big chance the SVS will sound better as it is tunable. -6db @ 20HZ Is comical for a sub as the sub still needs room gain to get flat. At which point you are hoping for the perfect amount of room gain. The SVS will allow you to adjust for room gain and a bit of boundary interference.
Rel’s high level input is a joke. No one with a lot of subwoofer experience would choose this connection. Yes it can work fine but why take the time to dial this very old method in.
In general I would buy the biggest sub you can, think 15” or 18” subs. Each drop in octave takes 4 times the displacement. Your mains have 3x9” drivers with a surface area of 189 square inches per speaker and I bet they have good throw distance too. A 15” subs is 175in/sq and an 18” subs is 254”in/sq. The less the sub has the work the better it will sound. You have top notch speakers that need top level subs imo.
if you like SVS I would look at the new Ultra 17 ($2500). They have a 30 or 60 day home trial.
If it were my money I would be looking at Rythmic 18 or JTR RS1 with an active crossover. I own the JTRs and have written about them if you want to see my detailed opinion. Big subs that play a wide bandwidth (10hz-200hz) will play 20-80hz with ease. Highpassing the main would take it up another level too.