My relatively new Amana APH1536 first floor package unit is not keeping my RH below 70%, particularly at night. My second floor fancy 20 SEER Am Std split system just barely gets it below 60% upstairs. Here are my house/HVAC specs:

First floor 1850 sq ft
Package unit: Amana 3 ton HP
CFM: 1100 (dipswitch setting B)
Return: 16" round flex, about 25' long, sealed with mastic
Grille: 20"x25", centrally located in home with CLEAN MERV 6 filter
Supply: 12 equal runs, nicely tapered toward far end from HP, sealed with mastic
Drain pan and condensation pipe in great shape w/ P-trap
T-stat is digital touch screen but no RH sensor
T-stat set at 75 degrees constantly
Fan is in Auto mode
Encapsulated crawlspace (CleanSpace)
Crawlspace Santa Fe Advance Dehu set at 50%
Energy star windows
Humidity range: 75% at 6AM 64% at 6PM
Temp range: 73 at 6am 75 at 6PM

Based on recommendations from my tech, I am considering adding a second 12" return duct and filter grille from my foyer area and then joining it to the existing return box. Then he would install a new 18" or 20" return duct from the existing return box to the package unit. Will this help my RH? Other suggestions???