Does the sub channel output of my DQ-61 have a low pass?

It does not.

Even though the channel 3 EQ only has the lower frequency knobs, it does not mean it can only accept a low pass signal, OR that it only outputs a low pass signal.

You can send a full frequency signal to channel 3, it will accept and pass thru whatever you send it.

The reason why there is only the low frequency controls on channel 3 is because 9 times out of 10 it will be used for a sub channel output.

DQ-61 product page