bipod72, you are getting lots of good advice & guidance here, so far. Generally speaking, your Marantz 50 watter is a fine amp but really doesn't have the juice (i.e. current) to make anything other than moderately priced 8 Ohm nominal load speakers sing. As such, if you're dead set on keeping that amp in this game, the Vandersteen would be a better match, comparatively speaking. However, at 86 dB, 1 meter with 2.83 volts input and 7 Ohms nominal impedance, your Marantz is still not a great match for them, either. You'd be better off, fidelity-wise, with the Monitor Audio Silver that panzwagn is suggesting and better off, still, bypassing your Marantz with that Adcom, providing the Adcom is still in good shape.
I've never done any serious seat-time with a Marantz 50 watter but have done so with the Adcom, many years ago. Much more recently (i.e. about 3 years ago), I did a considerable amount of serious seat-time with the previous version of the MA Silver 300 and the Vandersteen 2 when I was in the hunt for a speaker upgrade for my two-channel system. My ears liked both the MA and the Vandersteen. However, this involved using at least twice the juice of your Marantz. I didn't end up buying either one, but my ears definitely preferred the MA, comparatively. Those are simply amazing, at their price point and beyond! I haven't heard their newer upgraded version, but all the reviews I've read agree they are a tad better than their predecessors.
Good Luck!