I agree with Erik that connecting a REL or other sub to outputs of the Constellation preamp should work fine. And despite what REL may have told you connecting a sub or subs that accept speaker-level inputs to the amp should also be possible.
Since the amp has balanced outputs the sub's ground wire (the black wire in a REL speaker-level cable) should not be connected to any of the output terminals of the amp, and instead should be connected to a circuit ground point of the amp. With some designs connecting to a chassis screw will serve that purpose, although a loud hum might result in some cases. But what should almost certainly work well would be soldering the black wire from the sub to the ground sleeve of an RCA plug, leaving the plug's center pin unconnected, and inserting that plug into an unused RCA input connector of the amp. That would establish a suitable connection between the sub's circuit ground and the amp's circuit ground.
If one sub is used the red wire from the sub would then be connected to the amp's positive output terminal for one channel, and the yellow wire to the positive output terminal for the other channel. If two subs are used the red and yellow wires of the left channel sub would both be connected to the left channel positive output terminal of the amp, and the red and yellow wires of the right channel sub would both be connected to the right channel positive output terminal of the amp.
Good luck. Regards,
-- Al
Since the amp has balanced outputs the sub's ground wire (the black wire in a REL speaker-level cable) should not be connected to any of the output terminals of the amp, and instead should be connected to a circuit ground point of the amp. With some designs connecting to a chassis screw will serve that purpose, although a loud hum might result in some cases. But what should almost certainly work well would be soldering the black wire from the sub to the ground sleeve of an RCA plug, leaving the plug's center pin unconnected, and inserting that plug into an unused RCA input connector of the amp. That would establish a suitable connection between the sub's circuit ground and the amp's circuit ground.
If one sub is used the red wire from the sub would then be connected to the amp's positive output terminal for one channel, and the yellow wire to the positive output terminal for the other channel. If two subs are used the red and yellow wires of the left channel sub would both be connected to the left channel positive output terminal of the amp, and the red and yellow wires of the right channel sub would both be connected to the right channel positive output terminal of the amp.
Good luck. Regards,
-- Al