Like so many, I too was switching from ADAM to HEDD and here's my 2 cents.
As professional mixing & mastering engineer, accuracy, detail and sound stage is very important, what matters the most however is that the stuff you work on translates well. And for that I was looking into new speakers.
After hooking up the HEDDs for a first test, I almost fell of my chair. HOLY MOLY! Hello detail! The high resolution from top to bottom was insane. I never knew how much I was missing from my ADAMs.
I used it as closed port cabinet with low end extension mode on, linearizer on (duh).
For many days I just sat and listened to music until deep in the night (grateful to have a soundproof studio). I was hearing new things in music I had known for decades. I have heard quite many speakers in my time, but this one was special. Super detailed low and punchy and fast low end, clarity and forwardness in the mids and super detailed top end without fatiguing harshness.
The transient response and imaging was also something I had rarely heard before and probably never like this, even on much more expensive speakers!
Now here comes what to me kind of matters the most:
I of course also worked quite a lot on them, that's why I bought them in first place.
In the time I used the HEDDs I had exactly 0 revision requests for any work I had done, mixing/mastering/production/you name it...
Also the famous "car test" or in general listening to my work on other systems translated beautifully! That means a lot to me.
All in all my favorite speaker ever and I miss it dearly cause.... I sent it back to Thomann.. :P
I still want to try a few other speakers. But rest assured, if no other speaker can beat these HEDDs, which I assume to be very tough indeed,
I'll get the Type30 mk2 for the extra headroom instead and will never look back.
Don't get me wrong though. I honestly didn't feel like the Type20mk2 lacked anything. The low end, actually - the entire frequency spectrum was insane and the volume is much more than I'd actually need in a sensible working setting.
But for a big mastering room, I kind of want that extra woofer that the Type30mk2 would provide.
All in all I would say a sub is pretty much unnecessary with either of the two models.
I heard sub drops and sub bass better than ever and I used to have a sub here with a previous speaker setup, but never had such clear low end as with the HEDDs.
I could find only one thing to criticize; The hiss of the amps. Kind of surprised how hissy the speakers are (using the analog connection, my AES out is used for other things).
Definitely on the louder side of hisses.
I work in a big room, had them set up mid field/far field and still could hear the hiss.
But the moment anything is playing back, you don't really hear it anymore. Still kind of unexpectedly loud, though.
Instantly sold my ADAMs by the way. Wouldn't ever want to work on them again.
Music sounds just dull, 2 dimensional and lifeless on ADAM, even the more pricy models.
Of course that's just my opinion and you can have yours, but I can't be bothered with it anymore.
/my2cts