HARBETH: Their M30.1 – how much does it struggle with piano music?

I have a pair of M30.1s that are giving mixed results. Whilst superb with strings and vocals, they struggle with piano and seem to have significant coloration in the lower midrange, where the sound is distinctively 'tubby' and 'boxy'.

Do any Harbeth users have any thoughts about this? The speakers are in an 18ftx12ft room, firing along the room's length, with the cabinet sides about 1 meter from side walls and the cabinet rear about 70cm from the rear wall. I'd have though this is enough to avoid significant room effects with a speaker of this size and I do wonder if the size of the M30.1's cabinet means it has a resonance that sits at an uncomfortable point in the frequency range, resulting in the speakers struggling with percussive sounds.