You might want to find a Marantz stereo amp with the same output power as your quad gear. This would give you the two extra channels for a 5.1 setup. Get a quad amp and you would have eight channels. By using the same brand and vintage, hopefully you would get an amp with similar characteristics, and maybe even a matching faceplate! However no matter what you use, you need to run a phase test to determine if the added amp is in phase with the original gear. The point of this is that you want the speaker cones to be moving in unison when they share bass frequencies so that they don't cancel out the bass. This is also true of higher frequencies but with the bass, the effect is more noticeable. To run this test, follow the instructions below.
Take two speakers that are just alike (or known to be phased the same) and place them side by side. wire one to a channel on one amp and the other to a channel on the other amp. Make sure that phasing is observed. Take an RCA Y connector and connect the inputs of the two amplifier channels together. Set the two amplifiers at similar volume levels and put in some music from any source, using just one channel. Stand about 10 feet from the speakers and listen with both amplifiers on. Does the bass sound weak? Now have someone turn down the volume on one amp while you listen. Does the bass come back? If this is true, then one amp is out of phase with the other. That means that when you add the second amp you must wire the speakers positive terminal (red) to the negative terminal (black) on the amplifier and vice versa. You do this only on one of the amplifiers and to all channels on that amp. If the test conditions do not exist, then wire the speakers up normally.
This test should be run any time signals are run through dissimilar paths for the first time, such as through speakers of different brands. (when phase testing two dissimilar speakers, you would just use two channels of the same amp) If you are using amplified speakers for the extra channels, you will have to swap the connections on the quad amp because without opening the box, you will not have access to the connections to the amp output on the amplified speaker.
Incidently, the amp and speaker don't care about polarity. it doesn't matter if red on the amp is hooked to black or to red on the speaker, the amp will not burn up or anything like that. The only purpose of polarity markings on an amplifier or a speaker is to make sure that all channels are treated the same. Otherwise cancellation will occur.
The Quadfather
The Quadfather
