d) K812 is a perfected K701, which means fuller sound (approx +3dB for everything below 1kHz region) preserving K701's treble linearity, unlike warmer K7-variants.
It's also different in terms of presentation as it's slightly clearer and faster, leading to superior instrument separation/tracking.
Keep in mind that despite being slightly warmer, the K812 preserves the incissive, tool-like sound of K701 that can get sibilant or even strident if the recording leans towards that side.
In other words, K812 is not an easy going and relaxing headphone like the K712 can be.
High grade AKGs are known for being bright, they are all relatively bright...
K7XX is the one with more bass and likely the one that you'll find less bright. Not necessarily the most balanced, as all parts of the spectrum come into play when it comes to balance.
As I've said before, K7XX might fall into the slightly v-shaped camp, with raised bass, recessed upper mids/lowest treble (3-4kHz region) and relatively highlighted 6-8kHz region.