How much of a difference is there if I track bass/guitars thru something like a Heritage or Vintech emulation of a 1073 versus if I were to just plug straight to DI on my audio interface?

Is it really worth the $1000?

If something is worth the money depends on the person buying it and for what reason.
Sound wise it's not worth the money in my opinion.
In the end (final mix) I doubt any one can hear what pre you have used.
But some people also just love to say they own a 1073 and for that reason only it's worth the money for them.

Haha! Couldn't agree any more.

Well, let's leave money aside.
Does an external preamp 'fatten' the sound or cast some sort of magical spell ()to whatever's tracked thru them?

I currently track thru an Axe-Fx and for guitars, I find it amazing but I'm just not quite digging it for bass.

I read a ton of reviews on how some specific preamps 'sound amazing on bass', or 'on bass is where it shines', etc. and I can't quite help but get suckered in to that stuff

I'm still fairly new to a lot of outboard gear, but as a bassist myself and having gone through both interface DIs and something like my Summit tube pre, I have noticed a fairly significant difference. Oftentimes interface DIs are a little too clean, which is not inherently a bad thing, but with bass sometimes that's not necessarily what you want or need. Certain preamps will saturate in a pleasing way, or tighten/fill out the low end, just like a good bass amp will.

That said, I do think that you don't need to spend a grand on a nice preamp to get a great bass sound. Of course, the first step to any good bass track is a good performance. The rest is icing on the cake.