Speakers for newbie

I've always been a fan of Totem speakers and would recommend checking out their options. In particular, a pair of Arros would make excellent fronts which you could later combine with a set of Dreamcatchers or Mites and a Storm sub to make a full 5.1 system. Arros routinely come up used on Audiogon for around $700-800. If you can stretch your budget a bit more, a pair of used Hawks for you fronts would be even better.

Just because your amp is rated at 8 ohms does not necessarily mean it can't drive a 6 ohm or even a 4 ohm speaker safely--particularly if you're not planning to crank it up to maximum volume. It might be worth contacting Marantz (or a good dealer) directly to inquire whether there is truly threat of damage if you use lower impedance speakers. It might open up your speaker options considerably.