I checked with Reference Audio Mods. They told me they do not offer anything for the 861. They suggested I look into improving the sonics of the digital source (transport) if I wanted to get better sound out of the 861. Said lack of space inside the 861 limited what they could do.

2-Channel: Mer 800, Wadia 170i, Ipod, Berkeley DAC, Clayton M300 Amps, Martin Logan Spires
Multi-channel: Oppo BDP-83, Mer 800, HD621, Mer 861, Clayton/Rowland/Classe amps, Martin Logan Spires & Stage, Revel Embrace, REL Stentor