Hello Muzz and welcome,
hk integrated amps and receivers have never had problems driving 4-ohm loads, specifically the M80s, for decades, unlike some other brands. The hk's may run fairly warm/hot at high volume, but they don't shut down. I use one myself driving M80s.
The mythical "burn-in" to which you refer is actually a psycho-acoustical phenomenon, in which your brain/hearing system accommodates the sound of a new reproducer in your room. The actual speaker's frequency response and dispersion traits do not change over many years, unless it's been damaged or abused, or unless you leave it out in the cold.
The M80s are big and heavy, so to that extent they're harder to ideally position in smaller rooms than compact bookshelf speakers. But with some experimentation, they can sound wonderful. I have a pair of M80s and a pair of M22s on an A/B switching system in a room that's 19 x 13 x 9. The M22s are more ideally set up simply because they're compact, but the M80s still sound great. They'd sound even better if I could separate them more but my room is too cluttered to accommodate that.
Hope that helps.
Regards,
Alan
