That lower dome has always been something of a mystery to me. It wouldn't seem to correlate with the warp-to-impulse power conversion that they have been previously assigned to, given its distance from the impulse engine.
The
Miranda being a type of frigate/escort label does make some sense considering that the
Defiant, explicitly an escort, was partnered with them in the notorious 'Sacrifice of Angels' battle in which the Dominion weapons slice through the
Sitak and the
Majestic. There has been a trend to assume that 'escort' was a new label invented for the
Defiant since it was Not. A. Warship. but what if it wasn't a new term? What if 'escort' has always been the polite Starfleet term for a lean, mean gunboat, and the
Defiant is in fact the true successor to the
Miranda?
IIRC, the DS9 TM stat section (I know, I know - errors, errors

) also referred to
Miranda's rollbar phaser cannons as 'pulse phaser cannons.' Given that they didn't behave as such anywhere that we saw, perhaps another reason for the longevity of the
Miranda was the ability to graft
Defiant-type pulse phaser cannons into the same location? Alternatively, perhaps these cannons were of a more primitive pulse phaser type all along, and it was only the
Defiant that finally had 'real' pulse phasers? There does seem to be a correlation between these cannons' proximity to the warp nacelles on both the
Defiant and the
Miranda - so perhaps part of the magic of pulse phaser cannons is their need for direct warp plasma energy?
Then we don't have to call them megaphasers.