As an owner of RS850 Sig, RS450 Sig and RSC200 sig and a long time (20 years) follower of Danny, I know quite a bit about it.
Dick Pierce was the designer of the hardware and original crossover of the AV123 Rockets. The crossover design wasn’t great and AV123 asked Danny Richie if he could step in and redesign the crossover.
This became some of the Signature series speakers.
The 850 Sig, 450, and RSC200 Sig all had a completely redesigned Danny Richie crossover that was known to be much better designed, had significantly better parts, crossed over lower which greatly improved vertical off-axis and didn’t have tweeter rolloff like the original had.
The 850, RSS-300, RSC200, 750 and 750 Sig always had the Dick Pierce crossover. The only difference that I’m aware of is the 750 Sig changed the tweeter resister value to one that wasn’t as rolled off as that was a big complaint at the time.
Here’s a little talk from Danny about them: https://www.audiocircle.com/index.php?topic=186471.0
I’ve personally moved on to another AV123 product and own a full set of 4 AV123 X-Static and a X-Voce center which were 100% Danny Richie’s design and he still stocks replacement drivers to this day. Most of the GR-Research X-Series was available from AV123 at the time https://gr-research.com/x-series-3/