How many wpc am I using when listening at my normal level?

You are correct in your assumption of that you should measure 88dB as you inquired about. However, please keep in mind that the power calculations are not linear, but are logarithmic; meaning that they do not just go up or down like 1, 2, 3, 4, 5, ... they go up in factors just like all of those speaker impedance versus frequency response charts show with the vertical lines in groups of tens with decreasing horizontal distance from line to line as you increase frequency from left to right. Also, the impedance of your speaker at any given moment is determined by the frequency (ies) being reproduced at the time you measure, so the impedance measurement will vary substantially from moment to moment in almost all music genres. Even with a test tone CD set at a given dB reading at a set distance, if you change the test tone frequency substantially ... the resultant impedance will be different. Try it at 60Hz and then at 3KHz and then at 10KHz perhaps and see what your meter tells you if it does indeed measure true or accurate impedance. Most, or at least many, meters do not measure it accurately.
Unfortunately, Russell's reply will not produce an accurate power dissipation calculation due to your speaker's impedance being a nominally stated value; such as 8 Ohms or 6 Ohms, etc. That is a general or overall or net impedance and is not the actual impedance at the time you measure. While there are also other ways to measure and then calculate power dissipation, his way is correct minus the use of your speaker's nominal impedance value instead of the actual measured impedance based on the test frequency used at the time.
Also, and this just ocurred to me, if your speakers are of a type that are far more reactive than resistive, their impedance will vary all that much more based on the input test tone frequency. Try measuring the impedance of a transformer coupled ribbon tweeter with a nominal impedance of 8 Ohms where 7 Ohms is the Re, or DC resistance of the coil part of the motor assembly's Thiel - Small parameters. Then compare that to the impedance measurement of a woofer with very low Re and a very high Le, or inductive value in milliHenries of the coil, such as 4.2 mH. Obviously, you test both at the same test tone frequency for apple to apple testing methods. Magnepan speakers tend to be more stable in impedance even though they are low at a nominal value of 4 Ohms whereas B&W 802's may have a nominal impedance value of 8 Ohms, they are wildly varying in their impedance curve and the resultant more difficult load they present to the amplifier at any given moment. The Maggies would be easier to measure and therfore calculate an accurate power dissipation figure for a given moment. remember again that music is an electrical signal of varying amplitude and frequency over a given time period of a few portions of a second, so any measurement will and can only be for a given moment or so.
Any given high end class A power amplifier from a reputable brand, Pass Labs in particular for example, will easily be able to produce the SPL levels that you listen to at "normal" listening levels provided that normal to you is around the range you cited and not 128dB. Perhaps try the wonderful Parasound Halo JC-1 mono blocks that are class A amps up to either 10 WRMS or 25WRMS based on your setting of a switch for the bias. Maybe their Halo A21 amps would be a decent choice for you to get started with class A amps since they offer class A up to the first 7.5 WRMS as per Richard Schram's own comments to me before changing to a more typical class AB to keep the heat down. There are many good class A amps out there. Have fun with them and enjoy your music. God bless.
I hope that helps you.

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