No reason you can't use AR 3as with your Marantz -- power ratings are usually given based on an 8 ohm load, but as long as the impedance isn't too low (and if you are using only one pair at a time rather than paralleling two pairs) 4 ohms shouldn't be a problem. Some low end multichannel AV recievers might have a problem, but not a good quality stereo reciever. Speakers are given nominal impedance ratings, but their actual impedance varies with frequency, so the listed rating is more of a general interpretation of the impedance relative to frequency than it is a hard and fast number that has to be matched. With tube amps there can be a real benefit to using the correct tap off the transformer, but with solid state amps without output transformers its less important, as long as the speaker doesn't present too punishing a load. But the AR 3a has been used successfully with many, many solid state amps and recievers.
I can't pull up the linked site right now, but earlier it looked like the bass driver was the same Goodmans used in an oddball B&O sold in England -- the tax on imported large woofers was high in the UK at the time, so B&O designed a variant that could take a british driver, and (I think) a bigger one at that. It was more bass heavy than the usual B&O speaker -- in that sense no bad thing, but it seems to muddy the sound also.
