You can also place simple 1% MF resistors in series with the "hot" ends (the input side of the pots) of the volume control pots to cut down the gain. A value the same as the pot will decrease gain by 6dB, a value 4 times the value of the pot will decrease gain by 12dB, and a value 10 times the value of the pot will decrease gain by 20dB. It's a far less expensive, and more reliable modification than using in-line attenuators. Of course you can also simply lessen the gain of the preamp by modifying the resistor values around the tube, but that's way more complicated.

As far as implementing subs, I would run the output of the preamp into the line input of the subs (through the crossover in the sub), and the filtered line output of the sub into the power amps for the main speakers. With two subs, this gives stereo subwoofers.