Can I Use One Channel Of A Stereo Power Amp?

I think you can also. Currently I use a Krell KSA 250 stereo amp for the center channel on one side and the sub-woofer on the other side. Occasionaly I disconnect the sub with no apparent problems for the center channel.

I haven't had any problems doing this a few times over the years...there is no input signal into the extra channel, so no signal to output?

A single mono amp is a better option maybe? How about this:

http://www.audiogon.com/cgi-bin/cls.pl?ampstran&1259865296&/Marantz-700-Mono-Block-Like-ne

Note: Not mine, and I don't know the seller.

I agree on the single mono amp. Another alternative is a lower power amp that is bridgeable to provide 300 watts. Just be aware that many amps are not happy driving loads below 8 ohms when bridged, so make sure that either the amp chosen is capable of driving lower impedance loads, or the speaker impedance does not dip much below 8 ohms.

A mac 7106 is 100 wpc into 6 channels or bridged to 375 watts for 3 channels. Beat that? Cost is around $1250-$1500.