[ Its a Zobel network, named after Otto Zobel of Bell Labs, lots of recognizable names from that source, including Black, Nyquist, Johnson, Shannon, Bardeen+Shockley+Brattain, Hamming, Karnaugh, Kernighan+Richie+Thompson...]
You should add decoupling if its not present, otherwise you are likely to be on the cliff-edge of instability by just reducing that trimmer impedance a bit you haven't necessarily moved clear of the danger zone enough. Decoupling is not considered optional(!)