A few thing to consider.

1) Gain and maximum output level are unrelated
2) If other sources are OK, then check at the HDMI source component seting to see if some volume is lowered, late night automatic level control enabled or if downmixing/reencoding may be taking place in the player/TV/server.
3) Using an home made op amp module along multi-thousands dollars equipment is not good!
4) What is this 70 number you are refering to? dB SPL?
5) What volume position are you using when calibrating (the value of the minus dB displayed).