Yes, placement has an effect, up to a point. First, a standup, whether a spinet, a console, or a full-size upright has a soundboard. That soundboard resonates, and like all objects, it has a resonating frequency, with a bunch of associated overtones. In other words, not all standups were created equal. Because you are using one mic, you can't use mixing to blend brighter and darker positions on different mics. I don't know if you have more than one dynamic available, but if you do, try all of the ones you have available, and settle on the one that is closest to the sound you want.
OK, that said, there are 4 axes (multiples of axis, not a chopping tool) that are relevent. First is proximity. Assuming your mic is directional (most, but not all dynamics are), as you move the mic closer to the soundboard, bass will increase. At what distance this happens, and how abrupt or gradual the effect is varies by mic design. Generally, at 8" or more from the soundboard, proximity should not be an issue. At somewhere < 6", you'll hear it kick in. Just like positioning a cab mic, put on some cans, move the mic around, and listen.
The second axis is left-right. If you move the mic down the keyboard toward the left hand, bass will be boosted. That's sort of a no brainer, but- because bass wavelengths are very long, it has *less* of an effect than moving the mic toward the right hand, which will boost higher frequencies. This is also related to the keys that are used in the piece. If the piece uses all low notes, moving the mic to the high end of the soundboard won't do much.
The third axis is elevation. Although you are mic'ing the soundboard, go ahead and open the top. As you move the mic toward the open top, or even overhead, higher frequencies that don't make the soundboard resonate much will start to become more obvious. But- the open top is like the sound hole of a guitar, and the soundboard is like the top. If you get too close to the open top, especially if the mic is overhead, you will start to get excessive low frequency "boom", just like putting a mic on the soundhole of an acoustic guitar.
The fourth axis is directional placement. If you point the mic right at the source, the loudest frequencies, usually bass, will be augmented (look at the size of the strings-more mass on the bass side). As you turn the mic a little off-axis, *something* will happen, which is hard to predict, because it has a lot to do with the off-axis characteristics of the mic, the nature of the individual piano, and which end if the soundboard the mic is pointed toward. Again, put on some cans, and point the mic first toward one end, then the other, Record it, and say out loud."soft right, hard right, center, soft left, hard left," and have someone else play the piano while you are doing it. If you don't have another player, you will get to run around the piano a bit. In the end, you are better off listening to the recording on monitors to select placement, because headphones, although a useful tool, will lie to you about relative bass and treble.
My last 2 pieces of advice are these:
1. Over the years, most engineers have found that 2 to 4 mics works a hell of a lot better than one for this. In a studio situation, I use 2 overheads, and 2 on the soundboard, and I never know which ones I'll end up using (Yes, I've got a Wurlitzer console).
2. If you really are going to do this with one dynamic, either live or recorded, look around for a good price on an AKG D3800 tripower. You can find them for about $100 if you are patient, and it absolutely rocks on piano. It was designed as a high end vocal stage mic, but it's too damned unforgiving about placement for most singers. Pianos don't move much, and that makes the the D3800's narrow polar pattern (it's a hypercardioid), its abrupt proximity field, and its clarity ideal for that application. Good luck.-Richie
