Only subwoofer works at high volumes

It could be your receiver but you can try seeing if the Woofer has a input knob at the back, maybe this down, or maybe.if possible turn down the amount going to the woofer with your receiver.

Hey these type of problems can happen only wen the cables go wrong ... better replace the cables and give a try ...

Its probably the filter setting for speakers or speaker size/impedence or distance excerpt from page 12:
Link to manual

SIZE
Change to suit the speakers you have connected.
LARGE: For speakers that can reproduce a full sound range,
particularly the bass range below 100 Hz.
SMALL: For speakers that cannot adequately reproduce the bass
range. This setting is sufficient for most speakers if you are
using a subwoofer.
NONE: For speakers you haven’t connected (center or surround).
The factory settings are: Front: LARGE
Center and surround: SMALL
For the subwoofer, select YES if you have connected one (factory
setting), or NO if you have not.
DISTANCE
Enter the distance of the speakers from the seating position so that
the sound from all the speakers (except for the subwoofer) reaches
you at the same time.
You can select distances between 1.0 and 10.0 m at 0.1 m
intervals.
The factory settings are: Front and center: 3.0 m
Surround: 1.5 m
FILTER
This setting allows you to change the cut-off for bass output from
the front speakers. If you set the front speakers to “SMALL”, the
filter is set to 100 Hz. Raise the cut-off if the bass from the front
speakers is unsatisfactory so that this bass is output through the
subwoofer.
You can raise the cut-off from 100 Hz to either 150 Hz or 200 Hz.
DR COMP - Dynamic range compression
Change this setting to view software at low volume (such as late at
night) and maintain audio clarity. This setting works with Dolby
Digital software. It reduces the peak level in loud scenes without
affecting the sound field.
OFF: The software is played with the original dynamic range
(factory setting).
STANDARD: The level recommended by the producer of the
software for household viewing.
MAX: The maximum allowable compression (recommended for
night viewing).
D-INPUT - Digital input
Change these settings to suit the connections you have made to the
three digital input terminals, COAX, OPT1, and OPT2 (\ page 7),
so that the correct source is selected when you turn [INPUT
SELECTOR] (\ page 18).
The factory settings are: CD: COAX
DVD: OPT2
TV: OPT1