Output / Input Voltage Question

If I have a preamp that outputs 1.5v, but the amp's input is .5v, would this be the reason I don't have much headroom when I turn the volume up?  What is an ideal voltage match for preamp/amp combo?  Sorry if this is a dumb question.  Electrical stuff is not a strong point for me.
Combo is good, all you need is enough voltage to drive the amp and you have 3 times that. Not enough headroom, you need more amp.

P.S. 100wpc is not a lot of power. 
@russ69 My amp is a rebuilt and upgraded Quad 405 driving 4 ohm speakers.  Can't go past 40% volume without being way too loud.  I do have a small listening room though. 
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.5 volts is an unusually low input sensitivity for an amp. You can put an attenuator between the amp and preamp to fix the problem. This Schiit SYS will work!
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Can't go past 40% volume without being way too loud.

Why didn't you say so? All this voltage stuff is irrelevant. Your problem is you think that where the volume knob is pointed means anything at all. Relax. Where the volume knob is pointed means nothing at all. Or if you want you can tell people your amp is so incredibly powerful look how loud it is and the volume is barely cracked open. But seriously, where the knob is pointed is utterly irrelevant.   

This is in stark contrast to the numbers on the dial, which are incredibly important.  https://www.youtube.com/watch?v=4xgx4k83zzc  

That's a joke, son. (I have to say that because audiophiles have no sense of humor.)

@millercarbon LOL!!  I read all of these posts about voltage and impedance and I can't wrap my head around it.  Add speaker sensitivity into the mix and I'm completely lost. 
I'm in the same happy boat where noon is off and 5 o'clock is LOUD. Not a big deal except the less carbon track the signal has to pass through, the better. That's why a switched resistor type is better.
Beware audiophile tech talk, they will screw you up every time! Basically what happens, they will have you going round and round with a whole lot of stuff that is pretty much irrelevant. Not totally, and that is where they get you. Because every once in a while some of the stuff they talk about actually does matter. Problem is it takes you a good 10, 20 years of wasted time and effort to finally understand the vast majority of the time this stuff they go on and on about is well and truly irrelevant. Most of the time.

Like now. All that you have is an amp that does not require very much input voltage to drive it to full power. That is it. Period. Full stop. End of story. Nothing in audio matters less than where your volume knob is pointed. You get that now.

If you want you can take the time to study this stuff and when you understand it well enough then you will know ahead of time to expect the knob to be where it is. You will even be able to look at your source, be it CD or phono stage or whatever, and have a pretty good idea where the volume knob will be. It is actually pretty simple stuff once you get the hang of it.

Big picture though, keep it simple. Pay attention to speaker sensitivity. As long as that is 92dB or more you know you will always be able to find an amp to drive it nice and loud real easy without needing hundreds of watts. With turntables you want cartridge output of around .4mV or more in order to do the same, make it easy to find a phono stage. This is how to use specifications intelligently, to make your life easier not harder.
Do those two things and pretty much all the rest you can tell yourself, don’t sweat it. Eventually you may learn enough to understand why but in the meantime just tell yourself don’t sweat it.
What preamp do you have? 1.5V output from a preamp is very unusual. That's even lower than what most (if not all) digital source components provide which is around 2V for single ended outputs.
 As has been stated, if you have no issues with control over your volume, you are happy with your sonics, them you need not be concerned. 

Many amplifiers require over 1V to reach full output. Your amp requires only 0.5V, making it more sensitive to input voltage. Therefore you are attenuating much of the gain provided by the preamp, via the volume control. 
The OP is complaining about too much volume with a low setting on the volume control. That’s why I suggested an attenuator like the SYS!
But as usual the self proclaimed Guru of high end audio likes to show his wit with a ridiculous answer to the problem that the OP is having!
@donvito I'm not complaining about too much volume.  Just trying to learn the reasons why.  I've always had a hard time understanding all things electrical. And the Sys is backordered until May!
If the loud volume on a low setting does not bother you then don’t fret. It would bother me. BTW, I use the SYS to tame down the volume on my CDP to match the volume on my turntable!!
But as usual the self proclaimed Guru of high end audio likes to show his wit with a ridiculous answer to the problem that the OP is having! 
For the record, it is you - and others! - who have proclaimed me guru. I have never done so myself. Don't really consider myself a guru. Maybe I should, since so many others do. Be that as it may, all I did was help the OP with overwhelming complexity by showing how to make it all very simple. With my clumsy words. 

Now learn from a real guru. Seriously. Listen and learn.

 My amp is a rebuilt and upgraded Quad 405 driving 4 ohm speakers. Can't go past 40% volume without being way too loud. I do have a small listening room though.

Unfortunately this is really more of a historical issue.  Lets define a word:  gain.

Gain is the amount that a device increases the output based on the input.  It's really just a muliple, but we talk about it in decibels (dB), and is calculated using this formula:

    20 x log( Vout/Vin )

The point though is that the critical parts are the Voltage out / Voltage in.

For historical reasons, preamps tend to have way too much gain. This was so you could turn up the volume on really low volume sources.  This is why we generally can't turn the volume up past 10 O'clock except perhaps with a legacy system like an AM radio with a very faint signal.

Amps also have voltage gain, and I believe by convention it's 23 dB.

A preamp output rated as the absolute maximum output voltage.  Sometimes this is quite high, up to 15-20V. 

Amplifiers may be rated as either V needed to a certain output, or V needed to reach maximum.

But keep in mind that all of this is multiplication of the input, not a fixed value. :)