Hi jimr - not having owned a 950 I can't tell you whether the upgrade is worth it, but the 990's sound is sure an upgrade from my 18-year-old Adcom TPA-500, even on the phono input (which I seem to be the only one here who's still using). But on your mute questions: the 990's mute doesn't gradually decrease or increase volume when activated or de-activated, and you can change inputs but not adjust volume while muted.

Hope this helps.