I've done this exact upgrade- 24/96 5500s to 18-bit 6000s, and I would do again without hesitation.

There is a difference between 18-bit vs. 24/96, but it is more subtle. As in your case, I had to run my 24/96 center and 24/96 surrounds in 18-bit mode, but the overall change was worth it. To me the 6000s are much more refined, with better imaging, precision and much tighter bass. Once I experienced the 6000s, there was no going back and I eventually upgraded to 24/96 6000s and all was 24/96 again.

Feel free to PM me if you want to discuss more.

Theater: 861v4_LPS, 818v3, HD621, Oppo 203, Lumagen Pro, DSP8000SE, DSP5500C, SW1600x2, DSP5500, Epson 6040UB, PS4
Living room: 861v4_LPS, HD621, DSP8000SE, DSP5200C, Oppo 203, Lumagen 2143, 818v3, Sooloos/MC600 Bluesound Node
Bedroom: M80
office: G68D, DSP6000
Garage: DSP5000
Storage: M33s, DSP33s, DSP5k, 501, 518, M1500, G98DH, SW1600