I have a 70’s flatback 810 with 2005-era USA Ampeg 10s (USA Eminence made, Part # 86-032-01 10324A). They sound really good to my ears with my V4B-RI, but I don’t think they are a match to the originals.
I have heard that the Heritage 810 is as close as anyone has come to the original 810, but the drivers are special order from Ampeg, and it’s almost imposssible to get them unless you already own a Heritage cab.
If you don’t mind “close” then I think your choices listed above are good, and I have had a great experience with 12” Emenince Basslites (USA-made), so the 10” variant might be something to consider too.
An original V4 cab might suit as well, and likely cost less than the FlipTops unloaded cab (that’s what I put the Basslites in and I love it with my V4B). You can generally find them for $300 - $400 or so in OK to Good shape with speakers intact.
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Is that the 4x12 V4 cab?
I wonder how that sounds to Ampeg’s 810 design.