small mixer issue. Please help!!

DanLights

Santa Hat Forever
I have Nady 10-channel mixer (so small it's cute, pots are all super small) that I bought and use for playing live keyboard mp3 sequences from my Iphone (dividing the click track for the drummer's headphone monitors and the keyboards to the main console).

Now today I tried to use it with another purpose: for a mute rehearsal (aka with everything on headphones) which so much pleases my mother-in-law given the fact we rehearse at my girlfriend's house. This was only the guitarrist (my girlfriend) and me with no vox, and programmed drums/keyboards/click track from my Iphone (our band's 5th member since we kicked out our keyboard player). My problem is:

I tried plugging the guitar and the bass through the normal signal chain (effect pedals to small practice amp) and then plugging the amp into the mixer through the "headphones" out that both amps have. Plugging both into a line input each. this did not work. I had to turn the channel fader aaallll the way up to listen to a minimal amount of guitar/bass. So I tried getting the amp out of the signal chain, plugging from effect pedals straight to the same line in channel in the mixer, and it worked! good volume level for both guitar and bass, problem is distorted guitar sounded like shit without the amp (she uses a Boss ME-50, no amp simulation).

Could someone help me out on this? any idea why plugging from the headphones out to a line in gives me virtually 0 volume?