Lacking volume overall the volume control does increase and decrease the volume, but at the highest level it is still too quiet. Is there a way to fix or restore to original volume?

A number if things can be wrong with it, from the audil preamp to the the audio amplifier. Even as simple as a bad volume control. Does the actual noise change patterns when you move the volume?

Maybe try adjusting the MASTER level?

Good Luck

Other settings
Adjusting the master level You can adjust the level of the headphone and the stereo out by turning the MAIN VOLUME knob. The MAIN
VOLUME changes the analog signal level. This will also adjust the entire level of the MPC1000 digitally.
01. Press the [MODE] key and then the [PAD 10] (OTHER).
It switches to the OTHER mode.
02. Press the [F1] (GLOBAL) key.
The GLOBAL page will be displayed.
03. In the Master level field, set the level.
You will set the digital signal level, which will be sent out to the DA converter. The MPC1000 can output up to
32 sounds simultaneously. There is the great difference in the level when you play single sample and play 32
sounds. If you increase the value in the Mater level field, you can increase the entire level of the MPC1000 to
create more powerful sound with less noise. However, if you playback many sounds at the same time, the
level may be overloaded, which may cause distortion. If you decrease the value in the Master level field, you
can prevent distortion when you play many sounds at the same time. However, you cannot have a good
result in respect to the noise or punch of the sound. Set the appropriate value in the Master level field
depending on the application.

Blown or defective Headphone Amp IC or alternatively a defective Pot. Either way only AVID can fix it for you.

Outside parts and service is impossible.

Simple answer is yes.

Need more info: 1- does the volume control still change the level? 2-Does tone control still change the tone? 3- What happens when in the center position on pickup select switch? Put pickup switch in center position (both pickups active). Then take a coin and tap on the pickups. Does bridge pickup sound lower? Lastly remove rear electronics cover and inspect all wiring for something loose or disconnected.

In a few words I can clarify that the gain controls are affecting the volume of the corresponding input, so that you can set the slider at the proper level. In other words you use the gain controls in order to increase or decrease the audio level of the input so that the differences at the audio level from the sources (different recording levels, different cd players etc) will be eliminated.
In case you'd like the full user manual you can follow this official link. Click on your model and at the new window make out your name and your e-mail at the form and you will get the user manual.

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Stelios
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I had the same problem. This is how I fixed it:
1- Lay the MM6 upside down, remove the screws, open the back panel.
2- On your right side (the Volume PC board section), you will see component stand up (2 of them had 3 legs each, one of them is had more than 3 legs don't remember exactly) but 3 of them all had metal finish on their back. There are look like Power Transistor ( I do not have a schematic to be sure) but when these part get warm or hot (volume is only decrease when MM6 had been Turn-On for awhile), caused the volume to decreased.
3-Go to Radio Shack or other electronic stores, buy 3 HEAT SHRINK or you can make them your self (find- google to see what it look like, marterial...etc);
4- Use small screws and nuts, install these HEAT SHRINK into these 3 component then they will help these 3 components to stay cool all the time, therefore it will fixed the Volume decrease problem.
5- Yamaha should put HEAT SHRINK into these part when they designed.

TRY REPLACING VOLUME CONTROL BUTTON.

I'm having the problem too...I'm thinking that maybe getting the DC adapter MAY fix it though as stated in the Manual: "...The output level of the mini-KP will be smaller than its input level. This is because the attenuator circuit is connected at the input section, considering that the mini-KP is used on battery power." so maybe the dc connector will increase output volume?

If you are not a tech, I suggest you send it to one. If you are, after signal-dropout spray output transistors (one at a time) with component cooler (freeze spray). If still no results, go to the other transistors & IC's. Chances are that will pinpoint faulty component.