matching amps input voltage to source output volta

If I have a digital (cd) source that has a max output of only 0.5 volts driving a SS power amp directly ( cd has digital volume) which has a power rating of 100 watts at an input voltage of 2 volts.
So how much power is the amp actually capable of. Is it for example only 25 watts max ?( a quarter) or does it not work that way.

I ask because with these settings above my Wadia 381's volume levels can be used between the mid 80s to high 90's ( max is 100) which I know is good.
But alternatively I could either increase my cd's output voltage to 1 volt or lower my amps (Nagra PSA) input voltage to just 1 volt to get a better input /output voltage match and hence more available watts if needed for sudden peaks?
But then the volume on my Wadia goes down to between low 70s to mid 80's which means more digital bit loss .So which on paper at least is the better option. Thanks.
( please do not recommend a separate line stage an I have no more money!)
Bring your player as high as possible and your amp's input sensitivity as low as possible without eve going to the details for direct to amp setup.
So how much power is the amp actually capable of. Is it for example only 25 watts max ?( a quarter) or does it not work that way.
It doesn't work that way. The amp's output voltage (not power) is proportional to its input voltage. Per Ohm's Law and the definition of power:

Power = (Voltage squared)/Resistance.

Therefore the 100 watt specification (presumably into an 8 ohm resistive load) for a 2 volt input would become only (100/(4 squared)) = 100/16 = 6.25 watts for a 0.5 volt input.
So which on paper at least is the better option.
I doubt that it's possible to determine the best combination of settings other than experimentally. Among many relevant variables is the dynamic range of the music that you tend to listen to. Well recorded classical symphonic music, for example, will often have an enormous difference between the average power that is required and the power that is required for brief peaks, while for most rock music that difference will be very small in comparison. On the other hand, you will most likely listen to rock music at a higher average volume than the average volume at which you would listen to symphonic music. And the information loss resulting from the digital attenuator will vary depending on the internal design of the player. So it's all hard to predict without actually trying the various settings.

That said, it appears that your player is specified as using 24 bit internal processing, and having "21 bit resolution." And its digital volume control provides 100 steps of 0.5 db each. Since the material being played is only 16 bits, that suggests that you might not get significant information loss until the volume control is turned down below the half-way point.

-- Al
Thanks Al for the info.
If I'm right by changing my amps input voltage down to just 1 volt ( instead of 2) I'll get about 25 watts available of my amps 100watts rather than just the six I was currently getting. I think my Speakers (Harbeth M40.1s 84db) would appreciate a little more!.

Maybe I should also change my Wadia's output up to 1 volt to match the amps 1v input but I'm sure this would then put my volume range around the 50s-60s and wadia recommends 75+ as below will start 16 bit loss (although your posts suggests it could be a lower volume level before this happens.

Its interesting that most people and Wadia encourage owners to change the internal voltage settings to enable the volume control to stay as high as possible and above 75( which surely in most cases would be to a lower voltage than the amps input voltage) but do they realize that by doing so they are seriously reducing there amps output as in my case from 100 watts available to only 6! I certainly didn't.

As Harbeth recommends between 50-150 watts Maybe i need to get a 200-400 watt amp with an input voltage of 1 volt to enable me to still get my 50-100watts!
Pcoombs, since you are apparently using a balanced connection between the Wadia player and the Nagra power amp, an additional point that occurs to me is that on occasion I've seen inconsistencies between different manufacturers with respect to how balanced signal amplitudes are defined. For instance, if the output of the Wadia is defined based on the maximum voltage of EACH signal in the balanced signal pair, and if the input sensitivity of the Nagra is defined based on the instantaneous DIFFERENCE between the voltages of the two signals (which is what the receiving circuitry in the amp responds to), there would be a 6 db (four-fold) increase in the power capability that the numbers would seem to suggest. So it is conceivable that you might presently have 25W capability.

Re the 75% recommendation, that MIGHT be overly conservative, given that the player's internal resolution is 21 bits, and the resolution of the source material is 16 bits. Each bit contributes 6 db of resolution, so the 5 bit margin corresponds to 30 db. Turning the control down to 75%, i.e., by 25 steps of 0.5 db each, results in attenuation of 12.5 db, which means that only about 2 bits out of the 5 bit margin are discarded. Although other factors in the design might result in a greater sonic impact than those theoretically-based numbers would suggest.

Also, if you haven't already, see this Wadia paper.

-- Al
Thanks again Al for taking the time explaining all this. I'm not very technical minded .But yes I am using XLR cables mainly because the Nagra amp only has these available on back ( supposedly due to lack of space which does seem odd as they have generally recommended single ended conections!).

don't know if I'll be able to find out how Nagra defines its inputs as they don't always respond to questions but I'll try asking them .
All I know about the Wadia is that there is no change in volume levels (unusual) when switching between the rca and XLR outputs ( this has been confirmed from may other postings) whereas I know for most cd player etc the balanced output is 6db louder. But I think the Wadia is still a 'proper' balanced design but not certain.

One other question/ if what you suggested is possibly the case what amonut of powers is my Nagra possible capable of with the wadia at 0.5 v and the Nagra at 1 volt (we think 25 watts presently but maybe moreif what you said above is infact the case
)Thanks again Philip.
Hi Philip,

Whatever power capability you presently have, whether it is 6.25W or 25W, would increase by a factor of 4 if you changed the Nagra's sensitivity from 2 volts to 1. Doing that would double the output voltage of the amplifier for a given input, and power is proportional to voltage squared.

Best regards,
-- Al