I apologize, I was getting my brand names mixed up; I am actually using Aqua Coat grain filler, not Crystalac. It's the sanding sealer that is Crystalac.

Thank you for the link on how to mix the shellac flakes, it looks pretty straight forward. I did a test of the Aqua Coat on the front and sides of the bass body. The "face", or front, of the body was fairly heavily stained at the time, and it did reactivate the stain a bit, mostly due to me rubbing the filler in with my gloved fingers. I will avoid that and stick to using a plastic card edge from now on. However, the sides of the body (where the stain is at its present, permanent level of darkness) did not reactivate at all, and looks really good (it did darken a bit, as you mentioned it might). So, I think I'll stick with just using the Aqua Coat. For future builds, I will definitely put a protective layer of shellac in between the stain coats and filler, especially if I try to get fancy with the stain by using more than one colour.

Thanks so much for the advice, it is very helpful!