I ended up buying the Pro Ject Tube Box DS. Based on all of the reviews I've read online, the DS is more customizable and supposed to be a bit quieter than the Tube Box S. I've seen folks say that it's been offered at $399, but those deals don't seem to exist online anymore. I found it for $504 at Amazon which was at least $100 cheaper than anywhere else.
I'm not sure if the rest of my equipment is good enough to really take advantage of the DS, but I would like to upgrade my turntables (early 80s Sonys) and cartridges (MM to MC), so this should help with the future proofing.
I think I'm also going to stand pat with the stock tubes at the moment. I'll try to burn them in and see how they sound before I start rolling in new ones.