...I think I will clean out the cartridge with distilled water,
Yes. I use distilled water first to get most of the ink out of carts. Then I use a clear base for a final rinse.
then fill it with Amber base to flush. Or should I use your C6B?
Use C6B. The MIS amber base has a binder in it. You don't need or want that in a flush fluid.
The C6b is generic very generic, with no binders in it. I've used it successfully for a several years how with HP Z3200 PK. I assume the HP M has a similar chemistry. So, C6b appears to be very compatible with the HP inks.
People have also used the MIS amber base to make make an HP "LK" and "LLK" to reduce the bronzing of the HP pigments. It appears to work well. It does warm the HP PK-based inks a bit, but the bronze becomes silvery. See http://www.paulroark.com/BW-Info/1100-HpGO.pdf
With matte paper, I'd use C6b. For glossy paper, since I spray all serious glossy display prints, I'd still use the C6b. If I were displaying a glossy print without a protective Print Shield (or thicker) coating, then I might use the MIS glop to make the light colors. Note that I have not used glop (MIS amber base) with HP inks on an extended bases to see if there are long run issues.
Paul