Mic's for Piano

It is very difficult (read impossible) to fix the sorts of problem you are describing with filters... so you are on the right track.

Every piano is different, and any given piano will behave differently in different rooms, so a magical microphone selection/placement equation is not practical.

I'll tell you how I'd approach it, and you can season to taste...

First, if it sounds good in the room I'd try to record it in the room. Closely placed microphones are always a bit trickier to get right than room microphones, but sadly for us, room microphones are quite often impractical.

So now we're back to close microphones<G>...

Omni directional small capsule condensers are my first choice. For a smaller instrument I'd start with a crossed or near-coincident pair with the lid on half stick, and move the pair around until I found the sweet spot. My first choice would be a pair of Sennheiser MKH-408s, but that's because I already own them. Second guess would be a pair of Earthworks OMs, again from my locker.

Once again we do run the risk of running into reality, and omni-directional microphones may pick up room noise that you don't want... so now we need to look at cardiod microphones. You mentioned "tinny" so my first thought would be that the microphones are simply placed in some sub-optimal configuration. Pull them back, place them closer together, place them further apart, try lots of things and see if you can't tame the sound.

I'd also try muting one of the microphones, the problem you are describing could be caused by comb filtering that results from odd phase relationships between closely spaced microphones. If that solves the problem, and you still want the stereo pair use the 3:1 rule... the distance between the microphones needs to be about three time the distance from either microphone to the source. That can be a bit tricky on a baby grand<G>!

I don't know the specific models you are using, so again all I can do is reference my own locker. I'd start with a pair of Earthworks SR-77s. For reasons I still can't explain these things work brilliantly in closeup situations on acoustic instruments. I'd also try a pair of Audio Technica AT-4031s. They are noisey, but up close that is seldom an issue. I also love the Blue Dragonfly for this sort of work.

And don't get too stuck on small capsule or even condensor. I've used large capsule microphones and even dynamic microphones to record pianos with great (to my ears anyway) results. I also love a ribbon microphone pair on a really good piano in a really good space, but that is not a configuration that works well up close.

Have fun, experiment!!

It's a tough job, especially in the context of other players in the room.

Here's a really good thread about piano mic'ing with the lid closed (some good pics later in the thread):

http://www.gearslutz.com/board/remote-possibilities-acoustic-music-location-recording/97783-closed-lid-piano-miking-requested.html

Often times what I've liked has been something loose at the hammers, either omni or loose cardiod to pick up the attack, and a PZM taped to the lid (short stick) over the bass strings. This has been in a more controlled environment than a live stage though.

edit: One suggestion with the mics you already have is to not aim the 991 towards the bass side, but towards the hammers, in an effort to mitigate the comb filtering issue wst3 talks about. Pick up the attack with the 991 and the bass with the 990, and think of the pair less as a stereo pair and more of two halves of a mono signal. Also position the mics with a pair of headphones on, while monitoring the input signal to find the sweet spot.

Sounds like you might be experiencing comb-filtering (from reflections)...