It certainly sounds like a speaker upgrade would be the best way to go at this time. The Marantz components are fine for an entry level system, IMO. My personal favorite speakers in your price range are the Dynaudio Emit 10 or Emit 20. The Revel Concerta2 M16 is a good speaker as well, although I find them a bit on the bright side. The KEF Q350 has been getting good reviews, but I’ve not heard them, and I did not like their predecessor, the Q300. (I went through a lot of speakers before finding ones I liked, and found that for whatever reason, I didn’t enjoy a lot of well reviewed speakers, like the Q300, a few Wharfedales, the B & W 685, etc). Take your time and audition a lot of speakers, the discovery process is fun and enlightening.

As far as cables, I’d just use good quality 12 guage copper wire until you have the budget for more expensive components.

You should listen to plenty of speaker but an important thing to consider is the speakers SPL. This is reported in dB and the higher the number, the easier it is to drive the speaker. With that Marantz unit in your space, I would look for amps with an SPL rating of 88 dB or higher. You will struggle to drive any with a sensitivity much below that and the sound will be thin or hollow when you turn it up even to moderate levels. This would make the Focals you are looking at a nice choice since the SPL rating is 89.5.

This I would look at plenty of speakers but make sure you check the specs as that spec will make a noteworthy difference for you and may rule some speakers out.

I personally am not a huge believer in premium speaker cable. Maybe if you have really high end speakers like Wilsons, Magicos or Raidhos you can hear a difference but under $1500 is not going to be revealing enough to justify the added expense. I use 12 gauge monster cable in my ~$20K system and it sounds great.