6 dB/oct low pass is too slow for a sub. You will have it running up into the mid range. IMHO 12 dB/oct is the slowest I would use.
Why not just get a sub like a JL Audio with a fully active cross over in it that can be adjusted to suit? If you do not want an active crossover on the DQ 10s just put the right cap on the input of the amp (you have to know the exact input impedance) and passively role the DQ 10s off at 6 dB/oct? If I remember correctly Dalquist's own original cross over the DQ-LP1 had a 12 dB/oct adjustable low pass filter then they gave you a fist full of capacitors to install in your amp. (6 dB/oct high pass)
Why not just get a sub like a JL Audio with a fully active cross over in it that can be adjusted to suit? If you do not want an active crossover on the DQ 10s just put the right cap on the input of the amp (you have to know the exact input impedance) and passively role the DQ 10s off at 6 dB/oct? If I remember correctly Dalquist's own original cross over the DQ-LP1 had a 12 dB/oct adjustable low pass filter then they gave you a fist full of capacitors to install in your amp. (6 dB/oct high pass)