Honestly,
Combining multiple lines of Axanthic would only aid in compatibility as the alleles reside at different locuses. Since the function of this colour is to suppress the expression of yellow pigment; if you want brighter Axanthics a little outside help is needed. IMHO look to the bight morphs, Super Pastels, Fire/Sulfurs these mutations stay bright or even brighten with age. although they have yellow tones, the Axanthic gene pairs (reguardless of locus) would suppress that colour, giving you back a more visible result, no?