larger amp alt?

[ncc74656 2]

i have 2 batteries (group 31) and a 240A mechman alt. i just got my amps back today from repair and while driving (even on high way) the voltage drops. i got as low as 12.4 during most songs while at a high volume. lower volumes it performs much better than factory but im looking to stabilize my setup even at high volumes.  i did some drive testing today and my RPM sits at 650 idle, 1100 on most road driving and 1600 at 55Mph. when i turn O/D off the voltage seems to stabilize a bit more but still not perfect.

 

upgrade to a larger alt?

add another battery?

add a second alt?

 

thoughts?