OK, so I went ahead and bought the cab the other morning - itwas only Ģ69, so it's a bargain either way.
Turns out it's a pretty sweet combination, so I'm posting a brief description in case anyone has a similar question in future.
The Hartke 410XL has those aluminium cones, so it's pretty bright and sharp in tone. When it's down at ground level, though, you don't hear that as clearly. Stacked on top of the 115VX, however, those bright tones are far more noticeable, being closer to ear-level and all.
By comparison, the 115VX has paper cones, so its tones are warmer and softer and deeper. It complements the brighter sound of the 410XL really well, producing an overall sound that ranges from a deep, powerful bottom end up to some clean and punchy highs. I wasn't really expecting them to work together so well, so I'm pretty pleased.
Of course, I haven't gigged it yet, so I do need to hear what it sounds like outside the confines of the house, on stages of various sizes (and away from the sound of the walls and furniture shaking, lol). But it's a cautious thumbs-up at the moment...
The name that can be named is not the true name.