I’m going through old forum posts today with low (or zero) replies and high view counts. Hopefully I can answer some un-answered questions for people who happen on these threads later.
The DX7 and DX5 heads are almost identical but for a few key things. The DX7 takes a different ribbon cable and voltage than the DX5. The end result is the x880 heads just won’t work on the x800 printers. I found this out the hard way about a decade ago.