[Harp-L] FLAT!! But Maybe Not??

[Elizabeth Hess <TrackHarpL@xxxxxxxxxxxx>]

A band leader recently invited me up on stage to play harp on one song. 
 I'm a known quantity to him, and nothing stellar was expected of me -- 
just some tasteful 2-draw along with their blues groove.  (They said 
the name of the song, but the adrenaline rush prevents me from 
recalling it.)

They had already started as I was climbing the steps to the stage.  The 
bass player told me they were in the key of E.  I pulled out my A harp, 
stepped up to the vocal mic (that's what there was), started on 2-draw, 
and MY GOD, it sounded at least a quarter tone flat!  I was mortified.  
3-blow seemed flat, too.  I tried my B-flat harp, off mic, with a 
finger in one ear to hear it better, and darned if it didn't sound 
sharp.  It could not have been the case that the band -- which included 
a keyboard -- was collectively out of tune.  Nor did it seem likely 
that the bass player would have lied to me about the key they were 
playing in.  It didn't seem likely that two of my harps were out of 
tune, or that three reeds had decided to go in the space of one 
evening.  After some moments of distress, I decided that discretion was 
the better part of musicality, gave the band leader an apologetic look, 
folded up my harp case, and slunk off the stage.

Amateur that I am, I didn't check these harps' tuning the minute I got 
home.

Last night at my local jam, I got up to play, key of E, and the same 
thing happened.  The A harp sounded so flat I couldn't bring myself to 
try playing it.  I stood to the side and moved to the beat, sang one 
vocal without harp during the set (cf my New Year's resolutions, 
woo-hoo!), and steadfastly shook my head when they asked if I was going 
to play harp.  Pro that I am now, I stayed up until the end of the set, 
"listened rhythmically", and just served as "stage candy".  The set was 
fine without me, but I was completely flummoxed.  (P.S. no-one said my 
vocal was horribly off-key, so I don't think my ears are at fault.)

I am not a gear-head, but at the gig, I was playing into a vocal mic on 
a stand.  At the jam, I was playing through my green bullet and my HG2 
amp, with line out to the PA.

At home, I tried my A-harp and Bb-harp against my own keyboard, with 
and without the amp, and they were just fine.

So:  What might be the cause of this phenomenon, how might I prevent it 
(or what might I try), and if it happens again, should I just trust 
that my A harp is tuned correctly even if it sounds completely wrong to 
me through the sound system, and play, anyway?

The common element is sound going through the PA (vocal mic, line out), 
so obviously the first thing to try is to see if I can manage with just 
the amp, next time, and see how it goes.  But has this happened to 
anyone else?  Known phenomenon?  Known remedy?

Thanks in advance.
Elizabeth