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.