If you think you will eventually upgrade to all 24/96, it is probably easiest to sell the 18-bit version and just buy a 24/96 than trying to look for parts.

Also, nothing 'wrong' with 18-bit speakers at all, and while sounding different from the 24/96 versions, they are still enjoyable. One drawback however is the old source labels and they won't accept an upsampled signal.

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