Digital BW, The Print

I'using lyson quad black with epson 1290 Lyson gloss paper (profile 
lyson gloss)for 6 months. I spend money and time adjusting transfert 
curve and i haven't pretty good 21 step test. Without curve, 95% to 
100% looks like pure black (eye and scanner). The first thing i do is 
a "straight" curve 0-0 to 97% the result seems better in the dark 
zone but when i try to darken 0 to 40% zone (keeping the 50-100 the 
same), all the gamut is modified. The reason may be random working 
(ink, printer)?