Adding a single Repear 55hz to a 4x12

This question is on behalf of my son in college who plays late 60's/early 70's rock.  I bought an early 70's Ampeg V4 4x12 (on wheels) with the original Jensen CTS speakers to use at his fraternity house when he plays tailgates, etc.  While his acoustics sounds fantastic out of it, his electric guitars sound somewhat thin, especially outdoors.  I was thinking of adding one Reaper 55hz to add a little bottom end.  Is this ok, or should I go with 2 Reaper 55's instead and put them both on the bottom or some other alternative?

I need to stay with the "Live at Leeds" sound but with some mid bass to make up for the fact he's usually playing either solo or with another electric guitar.  He only uses the 4x12 and a vocal pa system for sound.