The target market for the 800A was near field monitoring above the meter bridge of a mixing desk, and dual concentric designs are really good for this, since the symmetrical sweet spot means you can sit or stand and move around whilst mixing without it making an obvious difference to the frequency response at the crossover frequency. Typical 2 way speakers used for this tend to get laid on their sides so that moving your head vertically doesn't affect the sound as much, and funnily enough the tannoy design that doesn't need this (dual concentric) still is designed with this in mind, with the logo being able to be swivelled between horizontal and vertical depending on how they are placed.
I agree, they are likely to go for around £400 if you sold them. They don't have much of a reputation (good or bad), the earlier tannoys like the LRMs being the collectable ones.
