I too cannot hear any difference (whereas I could on a previous Naim system, albeit very slight). My understanding is that any potential difference in noise floor due to processing more compressed files is removed from the equation in Sooloos by having the file uncompressed in the core then streamed to the endpoint. On the Naim forums at the time a popular thing to do was have the NAS decompress the file and then stream it to the NDX, with a similar result. I.e. On a Meridian system, don't worry about it. Personally I use the default 5 now.

HC: 861v4/ID40/LPS, DSP5000, DSP5000VC, DSP3100, SW1600, HD621, Oppo BDP-105, Sony VPL-HW55ES
2-Channel: Oppo BDP-105, TEAC VRDS-7/Trichord Clock 3/PSU, 551, Quad 303, Harbeth Compact 7ES-3, Kimber cables, Russ Andrews mains and supports.
Previous Meridian: 5000C, 5200SL, 7200.2, 6000, 562V.2, 568.2mm, 800v4, 808.2i, 818v2/v3, 861v8, 502