How many watts??

 The "OP" probably never gets past that 1st watt....
Almost all here fail to recognize just how much actual energy a transducer can output at (1) full watt. OR just how often all that they are  listening to normally?  All is supplied by that 1st incredible watt. "For many".
Even with speakers rated (85db) sensitivity!

millercarbon:

The great Robert Harley famously said, "If the first watt isn't any good why would you want 200 more of them?"    

This is wisdom.

Well I guess you did say "almost" all.

If I have a 80 watt stereo amp and I normally listen at quarter volume and never play at levels higher than this. Do I really need 80 watts could I use a 15 watt amp at half volume?

No, it depends on the input levels and your amp's gain. If your input level is very high, you could exceed your amplifiers output even when your volume knob is lower than half volume. It the input is very low, you could possibly not even come near your amplier's total output even with the volume turned all the way up.

There is no "golden ratio" between amps.

... if I have speakers that my mfg states needs min 50 watts to really make them sound good with my current amp rated at 80 watts but played at qtr volume does that mean I am not really getting the best from these speakers.

No, the manufacturers are engaging in a CYA exercise. The "best from your speakers" is what sounds the best to you. Don't sweat it, sit back and enjoy your sound. You are over thinking things.

Yup. What he said.

Your volume control is most likely logarithmic so you cannot use that as an indicator.  Go to Home Depot, Lowe’s or Amazon.  Order a volt meter.  Set the meter to AC voltage and measure the voltage across the speaker terminals while playing.  This will tell you how many volts you are using.  Amps or current drive is volts divide by speaker nominal resistance.  Power is amps time volts.  Most modern music is fairly compressed so 6dBs or a gain of 2 and 9dB or a gain of 2.8 will do.  A gain of 10 is 20dB which is way more than you will need.