98 Lexus ES300, Trying to Use Stock Sub

Hello, I recently installed a new headunit and am now trying to use an external amplifier to power the stock subwoofer. I have a Phoenix Gold QX 2180 that I’ve had lying around for 10 years. It was my first amplifier and I thought it would be easy to just wire it up considering I already have all the cables necessary. My problem now is this...I’ve just found that it is only 4 Ohm stable (at a Minimum in Bridged Mode) so I can’t bridge the connection for this sub. I see from the specs that the Amp is fine in 2 Ohm Stereo Mode. My question is this...Can I hook up the stock 2 Ohm subwoofer to a single channel on the stereo setting (either Left +/- OR Right +/-) and leave the second channel empty and run it fine? I would have the LP option on and set the x-over frequency? What will this do to my sub? Or will that not work? Is it safe? The Amp specs are: 4 ohm stereo @ 12/13.8 VDC 40/55 WRMS x 2 2 ohm stereo @ 13.8 VDC 90 WRMS x 2 4 ohm bridged @ 13.8 VDC 180 WRMS x 1 Crossover Frequencies 30Hz to 600Hz Crossover Slopes 12dB per Octave Thank you, Attached is a picture of the connections page of the manual.

Thought a of another question. What is the difference between Left and Right RCA signals for a subwoofer? Will they contain the exact same signal? If they are difference signals. Can you combine the L and R RCA’s with a joiner and plug into just a single channel? Ie: Left OR Right to get them as a mono signal? If they are the same: Can I just plug in the Right OR Left channel and run the 2 Ohm sub off a single channel? Thank you,

Hello! I've been looking around and determined that trying to, "Join" the RCA signals before the amp might damage my amplifier. Alternatively, I've found that you can buy (or make your own), stereo to Mono converter to place in line before the amp thereby safely combining the signals. RCA Stereo to Mono Is this necessary or as asked before can you just use a single RCA for the subwoofer?

Does the new head unit have dedicated subwoofer output????Hutson-

Yes. It has red and white. Kenwood emailed me back as said that it is a mono output so I'm going to just hook up a single channel and drive 2 Ohm stable off the amp.