I asked the EXH-6 preamp question of Zoom in a comment to their FB ad post and got the following response. So as indicated above, the preamps are the H6 preamps. It adds two tracks to the F4, but not to the F8.
"EXH-6 preamps are the same H6 preamps. When used with F4 it can provide 5th and 6th XLR/line inputs. With F8 it would just replace 1 and 2 (not as useful). For the F8, the mic capsule input is more for the rest of our interchangeable capsules."
Mics: akg c460 (ck61, ck63), c414buls, c568eb; at4049a, 4051a, 4053a, at853; josephson c42; neumann U87, km84i; beyer m130, m160, m500; aea r84; gefell m71, mt711s, m200, m201; sony c38; schoeps cmc6, CMBI (mk4, mk21, mk41, mk4v); sennheiser mkh40, md421, md431, md541
Pres: API, a-designs, pendulum, purple, millennia, gt, littlebox, tinybox, usbpre2, CA 9200, pipsqueak, pueblo pending
Cables: KCY, CMR, Naiant AKG actives, PFAs, asst. GAKables and Darktrain
Recorders/converters: dr680, m10, dr-2d, d50, dr70d, zoom f8, lynx aurora 8
Yeah, I'm an attorney, but everyone needs a day job!