Hedd, and more specific type 20 mk1, produce a lot of bass/low-mids but in a good way, so they really need a good room size and sitting between 1.5m to 2m. Otherwise, you'll feel the stereo image not quite right and you may struggle with balancing things.
They're quite similar to the Adams S3H, except that they sound much smoother and rounded in the hi-mids and top-end in comparison. sure the s3h can go a bit more louder.
But what I really love about T20 is the translation, seems perfect and I don't get any surprise when hearing the music in other speakers/locations. Specially if you mix a lot of acoustical stuff, you can get the tonal balance and character of the instruments nailed pretty quick with them.
And what I meant by clinical in my previous posts, is that the self distortion is really quite low, the speaker doesn't color the sound and you can hear and identify the ambiance of any recorded element very clearly. Also the balance and tonal doesn't change too much when you listening at low or high volumes. BUT they're not quite forward sounding, so you have to adjust to that, otherwise, your mixes may sound too forward and pokey.
I'm not sure if the mk2 performs any different, haven't tried them yet.