Your Sony receiver has a single RCA output labelled sub out. This will connect to the Lepai amp with a RCA cable and a Y splitter so you can drive both channels of the amp. You would then connect the amp speaker outs to the sub inputs. This will work but if you want better performance you would modify the sub by removing the internal crossover network. This duplicates what the sub out is doing which is feeding only bass to the sub.
You amp is design to produce full frequency range and you won't get the same performance when trying to force the amp to produce just the lower frequency. You might even be disappointed with the sound quality. This is because the sub channel will draw so much power may not have enough to give to the other channels.
Are you going to dedicate your receiver for sub woofer channel, use it as LFE or is use it mixed with other full range speakers? I suggest you get a separate amp even used one, is better than to mixed it with other full range speakers.