For me, I typically keep balls longer than most.  Because of this, when it comes time to replace them there really isn't an exact comparison available.  What I end up doing is starting from scratch and looking at what a manufacturer states their ball will do.  Too many times I see people with a 10-year old high-end ball and replace it with a current high-end ball and are amazed at the difference in reaction.

Lucky for you the Defiant Soul isn't that old.  I think you can get away with a similar ball in the same spot in the lineup as the Defiant Soul but don't be surprised if the difference in reaction is more pronounced that what you expect.

Plus, your stats are kinda typical, so I would be tempted to see what others around you with similar stats has to say about the new equipment.  You might even be able to borrow some. 

I'm having the same experience with my Totally Defiant. Aging gracefully is a great way to describe that ball. Now it's more of a somewhat stronger benchmark for me, but one that works well in any line I try when the oil dictates it's use. Probably my favorite ball ever.

If I look for direct replacements for a certain ball I usually try to find something in the same ballpark for core numbers. A coverstock with similar strength and/or shape is obviously ideal as well. To compare those across companies you can really only go off of what people are saying in reviews or what you see in person. For something on the high end like this it's a bit easier since you just look towards the high end equipment every other company has out, but when you get into the medium, medium-light, or medium-heavy coverstocks... good luck. 

Your specs aren't that far off from mine. I replaced the role the Totally Defiant had in my arsenal with a Mastermind Intellect and although it works well for what I want it to do the two are a bit different in terms of ball motions. Looking across what's out there right now a Snap Lock might be a fairly decent replacement. Maybe even a Crux Pearl. The Roto Grip line itself doesn't have anything out in that mold right now. Outside of the Storm family there's the Paradox Trilogy and Guru Limited. The Warrior Elite, Ultimate Nirvana, Trident, or Grudge Hybrid with some polish might be close too but all still likely being a bit stronger.

Based on numbers from bowlingthismonth, I would suggest the Swerve FX or Warrior(OG red pearl one). A lot of the other stuff on the market seems to me like it would roll a bit sooner or else weaker overall. Maybe the Mastermind Brainiac too.

I just watched a video with the Motiv Trident. The core numbers are identical to the TD, but the motion is so different.  Motiv covers intimidate me, haha