The way i see it,you have 3 options. 1) sell it 2) Find a second one 3) listen in mono. You are right. A mono amplifier should be used in pairs,that is(as i said above) you wish to listen in mono(through a single speaker).

Don't let anyone should on you
From personal experience, the fine art of mixing amps to create a magical stereo experience may be practiced without fear of insanity or wretched death ... just don't brag about it too loudly. Must? Tut!

Get yourself some old mono jazz LP's and enjoy!

If you are into SACD and what to do just LRC (C=center channel)you could get a Creek OBH remote volume control and send the center channel of your SACD player to the Creek then onto the Levinson amp, leaving your main 2 channel system intact. It appears that ones on Audiogon are going for $1500-$1900 each in pairs. If you could pony-up for two more you would be entering high end land for sure in 3 channel SACD.