Try REL subs and high level connection, no need to waste the RCAs on a sub.
Top 5 Reasons not to use REL "High Level" connection on the Lyngdorf TDA1120
1) REL's early marketing narrative (do they still say this?) for using the speaker level connection for adding a sub to your HiFi means: that the input signal voltage sent to the REL internal bass amp is created by using a very high impedance at the "high level" input on the REL. When you use this connection the REL's bass quality will "take-on" the dominant "sound qualities" of the Power Amp driving the main speakers. (so this rates an "Uh-Huh, right?)
2) Connecting your REL to the main amp outputs of the TDA1120 eliminates any use of the High Pass/Low Pass xover functionality of the Lyngdorf's bass management & room correction system while ignoring all the proven in-room acoustic and usable amplifier power benefits...
3) Ok, let's say you don't care about using the bass mgmt capabilities of the 1120 -- so you're gonna have your main speaker's Low Frequency extension "in-room" try to "blend" with the LP xover setting and in-room bass response of the REL? Room Perfect -- help!
Digression Alert: way back when REL was the darling High End "audiophlie consensus" subwoofer brand, their distirubtor (Sumiko?) used to promote a means of "room-tuning" the REL by starting with it buried in a corner, playing a bass passage on repeat, while listening and incrementally moving the REL out of the corner and closer-to/further-from the nearest main speaker/corner/back wall/side wall to judge various room Nodes/Nulls/Phase Cancellations -- I'm glad we have tools like Room Perfect, Dirac, etc., to work with now...
4) OK, so now you've resigned yourself to:
a) Ignore the bass mgmt capabilities of the 1120, and let "Room Perfect" sort out the blending anomalies of your main speakers and the REL sub via the REL high level input connection to the 1120 amplifier outputs (assuming it's electrically safe to do?).
b) Waiting to see if you can define the 1120 amplifier's inpact on the "bass character" of the REL's in room sound. (yes, I know, Uh-Huh...)
c) And since you have an advanced room correction system, you don't worry about how far the REL should be from the corner/back wall/side wall, etc.
5) Likely result: probably not too bad -- thanks to Room Perfect, but I'll wager it would be better by:
** Using the 1120 High Pass filter for the main amp/speakers: lower distortion in your main speaker upper bass/lower mids, more usable power for the limited output 1120 amplifier.
** Using the 1120 Low Pass for the REL: likely better Room Perfect integration/transition between sub and mains, likely better midbass and upper bass in-room sound quality.