You have a great set of classic studio monitors there.
I'm a little confused when you talk about multi-channel outputs. Are you refering to an Audio Video Receiver with mulitple amps like in a 5.1, 6.1 or 7.1 receiver (or possibly the multi-channel outputs on a DVD-Audio or SACD DVD player)?
Doesn't really make any difference. Any 2 channel stereo receiver (yes, they still make them and Denon makes several very good ones) will be fine as long as it has quality amps. Probably talking $300 minimum here.
If you buy an A/V receiver they all output in two channel stereo as well as multi-channel surround modes. Ones in the $500 range or above should have a good strong two channel amp output.
As far as your JBL's, they are quite sensitive (also called speaker efficiency). They have a spec of 91dB which I think is fairly conservative for this speaker. Also, the JBL's should be fairly close to their 8-ohm nominal impedence rating. All this means that you should have no problem driving them even with a fairly low powered receiver. Of course the better quality it is and strong low impedence power is always welcome. A good 100 watt per channel amp driving these speakers should be able to hit peaks in the 112dB range or more, very loud!
Any receiver with 50 watts into 8 ohms from 20-20,000 with less than .05% distortion, both channels driven should be great. If they show a 4 ohm spec with the same range it should have considerably more power than at 8 ohms (at least 70 watts or more). Many of the A/V receivers will run 100 watts per channel or way more in stereo.
Keep in mind your JBL's start to roll off in the 50Hz frequency area or so. This means for the really low bass in some of the current recordings a separate powered subwoofer would be a great match extending the bass flat to 30Hz or less. I thought some of the old farts that hang out here might enjoy the below info:
I recently had my Sansui Z-9000 die. I've had it since 1982. Also in 1982, I bought a nice pair of JBL 4312 speakers. I want to replace the Sansui with a mid-priced receiver and have a couple in mind. Do I need to be concerned about any problems that could happen if I drive my old school JBL's? (I know I need to push some power). Can I run these multi-channel outputs to only my pair of JBL's without ruining my receiver or should I connect speakers to all available channels? Hopefully these are not too stupid but I don't want to learn anything the hard way!