Here's a simple explanation:
THX Reference calls for 85db SPL in each satellite channel with headroom of +20db for peaks which mean a maximum SPL of 105db for each satellite channel. The .1 LFE channel has +30db of headroom for a maximum SPL of 115db. These are the SPLs AT THE LISTENING POSITION. In other words, it doesn't matter if your listening position is 6" away from the speakers or 60' away. The system is calibrated to produce those levels where the listener is seated.
Now HOW the system is calibrated to THX Reference varies. On systems with Audyssey room correction, the THX Reference level calibration is part of the automated calibration. The first measurement position is the most crucial and is placed at the MLP (Main Listening Position), usually the main centered seat equidistant from the Front Left and Right speakers). The calibration then sends out a series of full frequency sweeps (they're fast sweeps, so they sound like chirps). These Audyssey frequency sweeps were originally set using a -20db chirp (meant to produce 85db SPLs) in the early Audyssey equipped products, but after numerous customer complaints that the sweep 'chirps' were shockingly loud (most folks tended to perform the calibrations at night), Audyssey elected to decrease the sweep levels to -30db chirps which are meant to produce a 75db SPL at the MLP.
The Audyssey microphone measures these sweeps at the MLP and uses the measured SPL to set the trims/gains, i.e. if the mic measures the Right Front speaker at 83db at the MLP, it would set that speaker's trim to -8db. If it measured the Center Channel at 72db at the MLP, it would set THAT speaker's trims to +3db.
For folks NOT using an automated room EQ/calibration, you have to manually calibrate the levels yourself. In order to do this, you either
a) use the avr's internal test tones (check the manual or call up the manufacturer to confirm what SPL the test tones are emitted at, i.e. a -30db test tone means you measure the test tone at the MLP with an SPL meter and adjust the trims so you get 75db at the MLP)
b) use a test disc with calibrated tones at -20db (SPL levels should be adjusted to produce 85db at the MLP) or -30db (should be adjusted to produce 75db at the MLP). When using this method, you need to double check and confirm that the test disc is properly calibrated for the specific levels (some discs don't have calibrated level signals/tones).
Yes, the further you sit from a sound source, the lower the SPL will be comparatively, but that is the whole point of THX Reference level calibration. To normalize the SPL across varying setups in varying rooms.
Let me explain in further detail if you still don't understand:
Take the B&W 800d speakers. They have a 90db/w/m sensitivity. Typically, without room reinforcement (i.e. reflections), you can expect a 6db decrease in SPL for every doubling of distance.
So with this speaker, just for the ease of mathematics, let's take a listener distance of 4m (13.2 feet). At a distance of 1 meter, with 1 watt of power, the speaker is producing 90db SPLs. Double that distance to 2m and you only get 84db, double it again to 4m and you're now down to 78db. To be able to hit 105db at the 4m distance, you need to increase the SPLs by 27db. As a rule of thumb, it takes approximately double the power for each 3db increase, approximately 10X the power for a 10db increase. To hit 105db at 4m, a 90db/w/m speaker needs ~512 watts, and will be producing 117db at 1m. If you include the +3db amplifier headroom to ensure that the amp is not running into distortion, you need an amp capable of just over 1000 watt output.
Now, if for whatever reason, you elected to place these on either side of your seat so they were 6" from your ears, and you then properly calibrated them to THX Reference levels, they would STILL produce maximum peaks of 105db to your ears. The difference is that it only takes 1 watt of power to produce 105db at that distance (0.15m).
So for the last time, the speakers you're using and the distance you are away from them have NO BEARING WHATSOEVER if the system is properly calibrated to THX Reference.
As an aside, from looking at the above power calculations, you can see that it's impossible to use medium sensitivity speakers like these in commercial theaters where average seating distances in the middle of the theater would be maybe 60'/18m away. Those speakers would blow up if you tried to put the 10,000+ watts through them required to hit 105db that far away (not counting the +3db headroom which would mean 20,000+ watts). This is why commercial speakers have very high sensitivity. A 105db/w/m speaker would only need 324 watts at that distance, and 648 watts to include 3 db of headroom. Increase the speaker sensitivity to 110db and you only need 102 watts (204 watts with 3db of headroom).