The hammers are the parts of a piano that change the most, wear out fastest, and as much as possible should be eliminated from your initial impression of an instrument. Like tires on a car, they're part of a performance piano for only a relatively short period in that instrument's life. The timbre of a set of hammers can change dramatically with the humidity, and any wear or maintenance of the hammers will change the piano's voicing. To maintain the tone of a concert piano, hammers must be maintained as often as the piano is tuned. Hammer wear is directly related to the number of hours a piano is played and the intensity of that playing. Hammers can last for many years if the piano is played only three or four times a year, but concert instruments in constant use can require new hammers after only five concert seasons.
Hammers can be voiced using an arsenal of techniques, including needling, ironing, hardening, or filing the hammer felt, and fitting the hammers to the strings. A good voicer can make a piano sound more mellow without reducing its volume, or can make it sound brighter but without harshness. Even when selecting among different instruments of the same make and model, you will encounter pianos with a variety of timbres. Try not to be swayed by this; to judge an instrument, keep returning to the basic tonal capacity of its belly for sustain and volume, and thus for power and projection.
That said, hammers that are voiced too bright will make the piano sound thin in the treble, and may obscure what might otherwise be a beautiful-sounding instrument. If, during selection, a piano sounds a bit bright, you might ask the technician to quickly voice it down, or at least address the notes that appear to you to stick out above the others. If the piano sounds too mellow but has a long sustain, you can be assured that it will be very easy later on to brighten it. But I want to stress that any voicing changes made during the selection process should be minor — for best results, the piano must be fine-voiced in the hall in which it's to be used.
Of course, any process of selecting among several pianos should include playing them — after all, that's why a pianist has been given the task. If high-level playing weren't a necessary part of the process, a technician alone could do the job.
However, a concert piano should never be chosen solely on the basis of how its action feels. Why? Actions can be changed, and they will change on their own anyway. The action of a new piano will change as soon as it is played in and settled: the felt and leather pack down, altering the regulation dimensions. I can't tell you how many times I've heard pianists say that they chose a piano for its action, only to complain, several weeks after delivery, that the action doesn't feel like it used to. But such changes are normal, and easily reversed with a minor touch-up regulation.
In addition, actions are very malleable, and can be adjusted in many ways to accommodate the player. If you love the sound of a piano but don't like its feel, ask the attending technician if he or she can correct whatever you find objectionable. Some typical action-related complaints — all of which, within limits, can be corrected or changed — include unevenness (requires some touch-up regulation to compensate for uneven compression of felt and leather), difficulty playing pianissimo (let-off needs adjusting), and key travel too shallow or too deep (set by manufacturer, but slight adjustment is possible). In most situations, assuming the pianos have already been well prepped, these fine adjustments can be made in a short amount of time while you're trying another piano. In my experience, however, if the piano has been regulated consistently from note to note and is within a reasonable range of touchweight, a pianist of high caliber will usually have no problem adjusting to its action. Just remember that what is most important is the instrument's sound. Trying to separate that sound from your response to the instrument's action is perhaps the hardest part of selecting a piano.
Dynamic Range. Starting very softly, play a note in each section, repeating it with increasing loudness, and count how many discrete levels of volume you can produce. The more levels you can produce, the more expressively you will be able to play.
Play fortissimo and listen to what happens. Does the sound "top out" — that is, do you want the sound to get louder but can't get the instrument to give you any more? Does the sound "break up," getting ugly and harsh when loudest? Or can you play as loudly as you like without harshness?
Touchweight and Repetition. The touchweight of a concert grand action should be between 48 and 55 grams, and most modern grands easily fall into this range. If you like a piano's tone, its action can always be made a bit lighter or heavier. If an action is too light, however, problems can occur with rapidly repeated notes. Many good technicians can't play fast enough to make an action fail in fast repetition, and so have developed action measurements and tests to ensure that these problems never occur when a good pianist plays demanding repertoire. But now, in the selection process, is the time to test repetition with some really fast passages.