I've read with interest your reports on BCs, Dolby units, general tape knowledge and/or etc. Thanks for all the previous info.
As regards using the Seduction with the the 707, I've considered that already (I'm currently using one with my Technics). However, the problem I'm having is that connecting the Seduction directly to the heads, ala the Technics, would eliminate the auto-reverse feature of the Pioneer, which, frankly, I enjoy the hell out of with my 4 track tapes. Looking at the schematics shows that the possibility of hooking it up and still using the stock switching circuits is beyond my (modest) ee capabilities. Thus the thought to perhaps do some mods to the stock playback amp (new caps, etc).
FWIW, the 707 is a fun deck to use (as you say) as an exclusive 7" deck, and is readily available at around 250.00+/- in decent-to-good shape.
I have schematics available 'twould anyone like to take a gander and advise. (-: