I have original K6s. NAP 300DR really didn’t work in my lounge (16’ x 16’ - not large). Uncontrolled, booming bass, very unpleasant! Substituting a 250DR solved 95% of the issue, with the amp totally in control. My dealer spoke to Naim and Stewart Tyler (owner of ProAc), who suggested that course of action, as this was not the first time this issue had arisen. Final step was putting Stillpoints Ultra SS under speakers. Expensive, but an unbelievable transformation. Not only was bass perfection (to my ears) achieved, but the whole sound was given a significant uplift. Two years on and I’m still smiling every time I listen (which is for a minimum of 3 hours daily!).
Good luck.