QSK 2500 Amplifier Operation for SSB Mode using 'VOX' operation 
No, this is not a mistake!   Some hams have decided to use the QSK 2500 to eliminate the loud amplifier T/R relay sounds during SSB VOX operation.   (For SSB PTT, see below.)  In this new mode, the amp key signal from the transceiver keys the QSK 2500 through the AMP IN phono jack.  The QSK 2500 immediately keys the amplifier and keeps it keyed for 31 seconds.  During this time, the QSK unit performs all antenna switching for normal SSB VOX operation.  The SSB VOX user communicates without hearing the amp T/R relay clanging away.  The 31 sec amp key delay resets every time the user makes a transmission.  Consequently, the amp does not unkey until the user leaves a transmit gap longer than 31 sec. 
To enter the SSB VOX mode, the tip of a plug inserted into the 1/4" Key/Paddle input jack must be shorted to ground during QSK 2500 power up.  If the tip is shorted to ground at power up, the tip channel (i.e. dit channel) is disabled and the amplifier key delay is not read from the potentiometer but is set to a fixed value of 31 seconds.  If only SSB VOX operation is used, a 1/4' plug with the tip shorted to ground can be inserted into and left in the Key/Paddle input jack.  Then the QSK 2500 always powers up in SSB VOX mode.
If both QSK CW and SSB VOX modes are used, an easy way to enter SSB VOX mode is to hold the key closed or dit paddle closed during the first 2 seconds of power up.   Conversely, to enter CW QSK mode, do not close either the key or dit paddle during power up.  The delay time will be read from the potentiometer setting and the tip channel will be active.
QSK 2500 Amplifier Operation in SSB and AM Modes using 'PTT' operation

When using transceiver PTT operation for SSB or AM modes with an amplifier, the QSK 2500 should be set to normal CW QSK mode (not SSB VOX mode).   The QSK 2500 provides for receive operation  immediately upon release of the PTT.  The QSK 2500 keeps the amplifier keyed for the CW QSK time delay setting.  (default = 3 sec)   This means that the operator can switch modes between CW, SSB PTT, and AM PTT with no changes to the QSK 2500.