Currently the mod 6a doesn't exist as a set. I don't think imp exists.
Most people won't make the 6a when they can make the 5b.
I asked the same question a while back:
https://www.planetcalypsoforum.com/forums/showthread.php?279937-Adj-Imp-Mod-5B-Plates
I put together a guesstimate formula to assume the Imp/Mod prices.
I got my answer from the replies and bought a complete set on auction at +800 a plate for the adj 5b's. Hope this helps.
Thanks I didn't recall seeing that thread. Are those prices still current?
Also, do you dis-assemble the two 5b and put those parts back into the refiner with a bone and come out with an adj 5b, is that how it works, or is there an NPC who does it?
(NPC? What's this abbreviation for?)