A monoblock amp may or may not be a differential amp. A differential amp is also referred to as a fully balanced amp, or at least an amp having balanced outputs (meaning that the red and the black speaker terminals are both actively driven with signals, and neither is connected to amp ground).
The specs for the Audiopax 88 contain this statement:
ASTAT™ Asymmetrical Series-Twin Amplifier Topology uses two independent amplifier halves per monoblock whose dual output transformers are series connected for 30 watts of RMS output power
I'm not certain, but that leads me to believe that your amp has both speaker terminals actively driven, with neither terminal grounded. It would probably be a good idea to confirm that with the manufacturer or distributor.
In any case you won't go wrong by following the suggestion in the first part of Stan's post, when you get the second sub -- connect both yellow and red of the sub's cable to the amp's red speaker terminal, and black from the sub's cable to amp chassis. Do not connect sub black to amp black or you may damage your amp if it is in fact balanced.
(With one sub) If the mono blocks are together then I suppose that one hot would go to the hot on one amp and the other hot to the hot on the other amp and the ground to the ground terminal or chassis ground on either amp.
I suspect that this may produce a ground loop hum, although there is probably no harm in trying it. Otherwise you'll have to either use line level, or else connect to just one amp (as described above), which will result in just one channel of deep bass information being reproduced by the sub, rather than both channels summed together.
Depending on which version of the Quake you have, there is either a 0/180 phase switch, or a mode switch that provides 0/180 selection, so that will allow you to compensate for the Audiopax's phase inversion. Although depending on placement the 180 position may not necessarily be best -- go by ear as Stan suggests.