Michael,

I'd not use any joist near its max span limits.

Yeah, it was a weak design decision on my part... The majority of the house
has spans that are around 12', well within the capability of the 2x10 @ 16"
joists. Only the living and master bedroom have the 15' span. I approached
the span limits in those two rooms to keep things consistant. In hindsight,
I should have added another support or switched to larger joists in those
two rooms, but that does little good now.

If the new support beam doesn't give, you would expect a big change -
This makes me suspect that the support is actually not that solid.

Yeah, that's what I'm thinking too.

I'm gonna give it a couple of days to settle in a bit, then I'll check
things out better.

Anthony