I converted my DC's with a kit I got from
Adams Direct. I love it! Makes it easier to understand the radio and really cuts down on fatigue. Well recommended. Stall warners are generally high pitch and are not affected by ANR, since these mainly work in the low frequency region where passive noise attenuation does not do a good job.
Cheers
Ps: I do not have any interest in the company mentioned above, and assume the same kit is available from other sources in the UK.