Testing mix in mono

I just finished mixing a short track with piano, bass, drums and two vocals. A quiet piece with the vocals predominating and the piano as accompaniment. The mix sounds balanced on my JBLs.
 
I tried the trick I learned here on the forums and switched the interleave on the master bus to mono to check out portability.
 
The main issue as I see it, the piano disappeared slightly 1-2 dB into the background otherwise OK. My immediate instinct would be to lower the vocals, but upon further listening, the drums and vocals are louder than the piano so I would want to raise the piano. There is very little room reverb (sends from the drum bus and piano) on the track but I am wondering if this is revealing a reverb issue?
 
Any ideas or not to worry?
 
I will be posting the song in the song section tonight after checking out the bass levels in the car with a CD but thought I'd post this here now in case anyone has any thoughts.
 
Thanks, RK