Bruce Banner Leaves Yellowing from Veins and Midrib

Hi,
Anyone have this problem? Lower leaves turning yellow and seem to be affecting the vein and mid ribs. Edges are slightly less yellow.
Plant is 9 week autoflower about 2 weeks into flowering.
FFOF
Sungrow#4
Perlite
Tap Water 6.3-6.5
Top dress with Dr. Earth 4.4.4

I'm sure an expert will chime in here soon enough, to me though by the way it looks my first guess would be a sulfur deficiency? It appears to be a lockout of some sort. Have you tried anything yet, possibly a little CALMAG+?