I'm sorry I didn't answer your 6666.66 question.
My thoughts on this.
It is all just conjecture but it bears out better than charts sometimes especially on the low.
Always, always, always, remember BTC was and still is controlled by the miners. They are the consensus voters. Without them there is simply no bitcoin. This is how it was setup. So if you don’t understand mining and you are heavily invested you are dealing with a half deck of cards. Buyer beware.
Remember BCH would now be bitcoin if it were not for the miners. Everyone else wanted BCH to become bitcoin but the miners didn’t allow it.
Now for the 6666.66.
It took an exstream amount of effort and money to set the daily low of gdax to 6666.66 and it was no accident.
If you are familiar with the exchanges you will note how the price between exchanges changes instantly between exchanges, even though the buy and sell orders are different, the price is consistent between exchanges with very rare exceptions.
Myself and others believe the price is somehow manipulated by the exchanges in collaboration with an unknown entity possibly miners.
I believe along with other miners, which I will never name, the 6666.66 was a signal to the other exchanges not to allow the price to go below the 6666.66 level for the day. So all of the exchanges had to have bots in place fixed at this same price or slightly above so as to buy or sell, which ever was needed, to hold the price above 6666.66 low on gdax. This way it would remain fixed for the day.
I personally think this was done to shut down the rise in new mining operations and lower the rate of difficulty increase for the blockchain. It was allowed to stand for one day to see what effect it would have on mining.
I also believe it did not have a dramatic enough effect, only about 3.5 %, and they will creep it down until they get the increase down to less than 5% . It is currently sitting at 6.40%
The excuse for the price drop would be the result of the CTFC inquiry into exchange records and this kind of action is exactly what the CTFC investigation will be focused on, the price between exchanges being more than coincident.

Exchanges are not supposed to be trading with themselves at all let alone using bots to fix prices hi or low. Because of this investigation it may have been the last chance the exchanges will get to do it but it will not stop the major players from manipulation. They simply will have to work harder. They have enough money and coin to control the low price in a low volume market.
The high however is a completely separate matter and depends on public sentiment and volume. However the whales can still try and trigger a sell off if it is beneficial and these sell offs seem to coincide with bad news like the CTFC investigation.
Keep in mind all of this is conjecture as there is no way to prove it. But I believe it does quack and the CTFC also believes it may be going on.

Whenever you see the difficulty rate start to increase more than 10% twice in a row keep a close eye on your bitcoin or altcoin positions because this can start the big players, miners, warring with each other especially in a weak market where 3000 or 4000 coins can have huge impact.

They can easily part with coins and given any good reason they will. Remember a 10% increase in difficulty is a 10% loss for them no matter what the price of bitcoin is, unless they caused the increase in difficulty themselves. They are the ones loosing 10% so if they can stop it they will. If they can slow it down they will. This is why it is called the game.
You have to have stake in the game to play. Miners have the stakes in BTC, period. Everyone else is a spectator. No matter how much you paid to get in you have no say in the game because you have no mining power.