They are pretty inaccurate. Problem is that speaker sensitivity which is the key parameter for the calculator is a marketing number with no standards for measurements. It could be the sensitivity at 500 hz, 1000 Hz, the average of 250 to 1000 Hz, etc. In addition in rooms like yours with open floor plan, there is less room loading and hence the losses will likely double. Keep in mind that when someone says the calculator is a few db off, at 3 db you compute the power requirement wrong by a factor of 2! So 30 watts may be 60 watts.
As to the long thread, it got long because folks there, who are also posting here, assume idealized amplifiers, and not how they are designed and work in real life. Passive bi-amping can reduce distortion level. You would need to determine if in your case it does that or not. Using your current system without bi-amping keep turning up the volume while listening to a dynamic music. Do you hear the sound fidelity change at some point on the volume control? Perhaps becoming more shrill or thin? If so, your amp is running out of juice no matter what the formula or a thousand posts here say. Try passive bi-amping and run the same experiment again. See if that point of fidelity change has moved up, hopefully beyond the point that you want to listen.
Thanks for replying! Interesting. I was thinking to myself that the SPL calculator may have given misleading information as 10 watts to hit 98 dB at a 3 meter distance just seemed wrong. My room is quite large. So the calculator can't take that into account. I have no idea how much power is actually being drawn from the amp then, but you suggest I should just double the calculated power to be on the safe side.
So that would mean 20 watts 8 ohms, or 40 watts into 4 ohms or 60 watts into 3 ohms etc assuming 98 dB SPL. But it also doesn't tell me at what frequency I'm hitting. So again, the results could be totally off. If I needed 98 dB peaks and there was high energy 40 Hz signal there, I'm guessing here that I would need a hell of a lot more power than 20 watts.
So if the calculator can't take these important variables into account then what good are these calculators? Because it would seem they are just giving misleading information based on insufficient data.