The volume is a result of power vs. speaker power handling & sensitivity. Im not sure what the sensitivity is of your MB Quarts, but if it's low, you won't get as much volume. To overcome this, you'd simply need more power from an outboard amplifier. I'm not advocating getting rid of the MBQ's though. Knowing part/model numbers of your HU and speakers would help.
The head unit grounding location has little to do with overall volume, but it could afffect system performance. I've really no idea how grounding the HU to itself would affect performance, but electricity is a strange animal. It is highly recommended to ground the heat unit to metal attached to the car's frame. A direct ground to the firewall, which is part of the frame, is best--no longer than 18" & preferably the shorter the better. Another piece of metal will work, but the ground wire from the harness to the head unit itself isn't an ideal ground. If you must extend the harness ground wire to make a good connection, be sure to solder the hell out of it and either tape the connection well or use heat shrink tubing to cover the connection. Metal to metal--make sure paint or any other insulator is not in the way--you can paint over the ground later when the connection is secure and you wish to prevent any rust. The HU ground is the most important ground in the entire system, so spending some time trying various locations is time well spent. Make sure the ground is absolutely secure & tight. The vast majority of ground problems I've seen are from owners who twist wires together...the other issues are from noisy, unshielded components (like cheap amps, EQ's, crossovers, etc.) and poor grounds.