amp running? volts

Yes, that's called "setting the gain too high".  I take it you are relying on the voltage numbers printed on the amp to set the gain?  They are there to give an indication of "sorta" how the input sensitivity is configured, but using those numbers alone will not get your gain set correctly.

Some good threads have been written in the Car Audio forum on setting gain.  Start with this one, then this one, then this, and then this.  Basically, you must go through the process of first finding your deck's clipping point, and then when you find its highest clean output level you adjust the input sensitivity of the amp so that it reaches its highest clean output level.  Both units will reach their maximum clean power at the same time.  You are matching the amp to the deck, which is referred to as setting the gain.  The test tone is important, and I use the program you will see in one of those threads to make test tone CDs.  There are some others, too, and you can find them with a google search for "sine wave test tones".

When you get yourself all set up, it's a cool thing to do because when you're done you will know that you will never send a clipped signal to your subs (or other speakers) as long as you don't turn up the deck past that "magic" number.

The marked positions of the gain control give you an idea of this:  what the amp expects of the input signal. Set to 1.5 volts means the amp is set to reach full output with 1.5 volts of signal.  If your deck actually provides more signal at 2 volts, the gain on the amp will thus be set too high.  The amp reached full power at 1.5 volts and the deck is feeding it additional signal.  That can only mean that the amplifier will go into clipping and send damaging power to the subs when you are at full deck volume.  If you are relying solely on those markings, use the 2.5 or 2.0 volt marking instead.  But as I tried to make clear above, you are not actually setting the gain properly this way.

When, or if, you ever decide to learn how, with the help of those links, and then do it, you will realize what a fine line there is between good signal and clipped.  The difference in the touch of the gains between good and bad is so slight you wouldn't be able to do the same thing by looking at printed markings.  Both the deck and the amp have to be set, not just the amp.  When you hear the change in pitch and tone of the sine wave you will immediately know it has gone to clipping...it's a very distict change in sound, but very much harder to hear when you're listening to one of your music sources.

BTW, if  you think you need the bass boost then set it to one of the lower settings.  Like no more than a +3 db boost.  3 db is twice the power to that freq range the boost is set at.  And it whacks out your gain setting if you turn it after you've set the gain.  It's best if your gear is good enough to begin with that you don't need to use the boost, but if you do use it, set the boost first before setting the gain and then leave it alone.

I just want to reemphasize what Steve has already posted: the markings on the amp's gain controls are only there as a reference, just like the voltage output of your headunit.  Just because it says 1.5 volts on the gain doesn't necessarily make it so, neither does the deck's claim of 2 volts output necessarily make it so either (it may be higher or lower).  With your car running, your RCA's may recieve a higher voltage signal than when your car is not running because the deck is getting a higher voltage (around 14 volts as opposed to 12 volts).  This is all to say that there are many variables involved, and simply setting the gain based on numbers on the side of the amp is a crapshoot.

Do a search in here on gain setting as Steve suggested and learn the proper ways to do so.  We are not slamming you, so please don't take it that way.  We simply want to make sure you learn properly.