No worries! The Decware CSP3 preamp can put out up to 30v, so not need to worry about 3v. The only way this would cause damage (to my knowledge) is overdriving your speakers, which is going to be very loud before it happens.

What you've discovered is an aspect of riding the gain. Feed your amp a little more voltage (3v in your case) and you don't have to turn up the amp as much to get the same volume as you would with feeding it 1.5v, for example. Many times, greater voltage in can sound denser and weightier.

The standard CD player out is 2v and many DACs put out more. Some up to 5v. In short, you will still control volume and as long as you don't blast yourself out of the room and destroy your speakers, harm will not come by feeding your amp a little extra input voltage.

I say try all 3 settings on your DAC and choose the one you think sounds best.

You are correct on nominal voltage. I should have mentioned that. This means 1.5v is really the minimum the amp needs to sound full. Less than that can make the amp sound thin. More than that will make it denser as explained before. Either way, the amp is unharmed but the sound is effected and the volume knob must be adjusted to get the volume you want.

In general I do prefer a preamp because of the greater control you have over the gain and sound, but as long as your source provides the nominal input voltage a preamp is not required. It should sound good, but a preamp can make it sound even better.

There are some that prefer to keep the audio chain as simple as possible and do not use a preamp. I have tried both ways and often go without a preamp if the source provides enough voltage. I have a CSP2+ preamp, however, for those times it is either needed, or I decide I want to see if things can be improved. Often, they can.