It sounds like you don't need quite so much cabinet! The ampeg 410HE is pretty loud, and doubled up with another cab is VERY loud (I use two of them in an irresponsibly loud band and, like you, rarely turn up my master volume past 2 or 3 on the dial). It sounds like this configuration would be way, way too loud for what you are doing.
The GK Neo 212 is going to sound very different from your sealed 410HE (and may not be much quieter). I notice you said things sound beautiful right now, so swapping these units may change it unfavorably to your ear. Or maybe you'll love it
Just for kicks, my process for approaching this would be something like:
Fusion 800 + Ampeg 410HE alone:
1. Do I like how it sounds?
-YES: go to question 2
-NO: end. Try different cab!
2. Is it loud enough?
-YES: I like how it sounds and it is loud enough. I'm done!
-NO: Look for a similar sounding, louder cabinet arrangement. (610 HLF, Berg NV610, pair of SVT212AV's all come to mind while being completely insensitive to budget).
If none of this works, go to scenario 2:
Fusion 800 + Orange 15 alone. If you like how it sounds, stick with it. If more horsepower is needed, get another Orange 15. I haven't any experience with this particular cabinet and driver, however typically 15's push less air than a 410 (I know, driver sensitivity and efficiency count but I can't be bothered to do the research). If this cab's tone makes you happy, 2 of them may be just the ticket to having enough volume on tap in case things get loud without creating a runaway train issue.
My own experience with the 410HE is that it sounds a little bit thin on its own, but fattens up to super greatness when doubled up. I'm sure the Orange 15 would change a bit in character if you added another, but generally if you double up on more of what you already like the result will be goodly.
Look forward to hearing how you solve this!