Where did you get the March 18 7 cents NAV number from? My only source for historical NAV is Yahoo (rarely reliable) and they show 40 cents for March 18.
The NAV number is only reported once a day, after the market closes, so there's no range between the high and low - there's just 1 number for the day.
And DSE hasn't had a tax asset in years. It has a huge capital loss and ordinary loss carryover, which would normally create a deferred tax asset. But they never thought they could utilize the carryovers so they set up a valuation allownce against the losses, taking the deferred tax asset to zero.
What killed the NAV disproportionately (same every one of the damned MLP CEFs) is leverage. At November 2019 (the last annual balance sheet date) DSE had 40% leverage - 19% debt and 21% preferreds. They can't get ri of that leverage on a moment's notice. For instance, they have to issue a formal call to redeem the preferreds, wait some specific number of days, and then pay them off. So as MLPs tanked in early March, the gross asset value was shrinking rapidly but the debt/preferreds couldn't shrink quite as fast. That timing gap killed the NAV for everyone that didn't reduce leverage quickly enough. Which is basically everyone. The management fees are based on total assets, so there's an incentive to maintain the leverage as long as possible. I think that is why DSE and the others underperformed the index.
There is an unleveraged ETF, AMZA, and at one point it dropped almost 40% this year. Terrible result, but only about half the damage done to the CEFs. I think leverage was the killer, not the tax assets getting written off.