New mains to complement Usher 5.1 setup?

I have an Usher 5.1 setup with S-520 mains and surrounds, S-525 center, and SW-210 sub, currently driven by a Cambridge Audio CXR120 AVR. We will move in a few months to a house with a deeper AV area and I am thinking of expanding to 7.1. The easiest and surest move is to just buy a third pair of S-520s. But, I am open to upgrading the mains to floor standing speakers that will both play nicely with the existing Usher speakers for surround/theater, and provide an upgrade when listening to music in stereo. My budget will be the $2K range, new or used. The speakers will likely continue to be driven by a mid-fi amp for the foreseeable future, so nothing too revealing of amp flaws. Thanks for guidance and suggestions.
Thanks for this.  $3K is a stretch at the moment.  There is also some concern for placement, as this is an AV setup in a living room/den, not a dedicated room, and I read that the MD2 is somewhat fickle about location, and requires lots of room to breath.  I may need something more forgiving about location (and less expensive.)
These are excellent all-around speakers with a similar sonic signature to Usher...

If you prefer new the LSA-10 Signatures are available here from the manufacturer for $1200/pr and would also be excellent.  Just a couple more ideas...

Perhaps consider the N6311 or N6361. They are based off the CP-6 series and represent great values.
Usher N-6361 looks like an intriguing option.  Any thoughts on GoldenEar Triton 3 / 3+ ?
Uh, forgot to mention the obvious upgrade to the Usher SD-500 that goes for about $2k/pr new.  Duh.  It’s pretty impressive to be able to get their flagship diamond tweeter at this price level, but then it’s Usher so not really surprised.