SVS SUB BETTER WITH LOW PASS FILTER OFF

One things for sure… I need to see pictures of the yurt hifi room!

I would think your room would be almost 100% null free other than ground bounce. the La scala having very poor bass extension I am sure aids in the higher “crossover” point. 
 

I find my personal subs always lack a bit of detail compared to my mains above 100hz. I also find the sound stage with deep voices can get a little strange when using high crossovers. With that being said I will say a lot of subs play very clean all the way up to 200hz.  

I have the Arendal subwoofer and set the bypass at 10hz -15 hz above the floor of my main floorstander speakers that go down to 35hz. I read somewhere that 10hz above the low was a good place to start. I too switch the low pass 12hz filter off, as that works for me too. Saying that I use very little db gain, about 2.5db to get a very smooth integration. I agree doing it by ear even though I can do it through an app strung in my listening position is not as good as having some form of room correction, but in my case the room, is the room, is the room!

Glad you found the settings that work for you in your unique room.  Agree we need to see pictures your room sounds cool.  I have a pair of SVS SB1000 Pros.  My 2 preamp sub outs are also fixed with no adjustments available so like you I have the crossovers setting on the SVS app for each sub set to “off”.  It sounds great.  The one thing I will adjust is the sub volume as needed on the app depending on the music I’m listening to.  If the bass starts to overwhelm I’ll turn it down a few dbs or occasionally up.  Usually have them set to -12 db. As instructed in the SVS manual. 

The most vexing and confounding problem I know of is how different speakers measure once in a real room, and how different each room is.  I'd go mad if I tried setting up a subwoofer by specs alone.  Some may argue that I "went mad" instead, but potato potatoe.

Also, hot tip:  If adding a sub to ported mains try sealing the main speakers.  That often leads to easier integration and less distortion.