[1] In order to hear how deep can the bass go on a system I often use "Anything's possible" from "Zero dB". On a lot of systems the lowest tones of the bassline don't even come out, on other certain frequencies are exaggerated or shifted.
[2] As said above it's all relative to the other systems I know and nowhere near a scientific method of measurement.
[1] Obviously a system incapable of producing any of that deep bass is not a good system but beyond this basic determination of a poor or decent system, we're stuck. Assuming a decent system which can reproduce that low bass, the qualities of that low bass; it's punchiness, balance with the rest of the mix, etc., are unknown. Maybe that low bass is supposed to be barely audible, maybe it's supposed to sound exaggerated/dominant or maybe a particular point in-between? The only reference you have is your personal preference of how much (and other qualities of the low bass) you like.
[2] It's relative first to your personal preference and then to the other systems you know and which of them get closer or further away from your preference. Let's say the low bass is supposed to be barely audible relative to the rest of the mix but you prefer it louder. A particularly good system, which accurately reproduces that bass balance/level, would sound less good to you than an inaccurate system which exaggerates it.
[1] For me, I listen to hear instruments - Does that sound like a piano? ...
[1a] Does her voice pierce my ears? Etc, Etc.
[2] The vast majority of times, its the recording/mixing process that will determine the answer.
1. Again, that doesn't tell us much, as any competent system should make it sound like a piano. Going a step further though, what is that particular piano supposed to sound like? A Bosendorfer sounds different to a Steinway, a Steinway concert grand sounds different to a baby grand, the room in which the piano is played makes a dramatic difference to how it sounds, as does how it's played and defining all of that is how it's recorded, with what and where. So, you have no idea what a particular piano recording is supposed to sound like beyond the basic fact that it is a piano and then your preference of how you think or would like it to sound.
1a. Is her voice supposed to pierce your ears and if so, by how much?
2. That's a bit of a strange statement because ALL the time it's the recording/mixing (and mastering) process that determines all the aspects of the sound.
BTW, Waldrep is very knowledgeable, very skilled at recording and is the most honest (least full of bull*) of anyone I've come across who markets hires products (content or equipment). Occasionally though, what he says is a bit dodgy, as at the end of the day he is trying to hammer a square peg into a round hole. He's certainly near or at the top of the list of most recommended, particularly for his surround productions.
G