AVS forum has a strong bias towards all receivers sounding the same, all amps sounding the same, cables are snake oil and so on. Unless they've changed their ways, it's best to know the bias going in.
It sounds like the OP has been running a 3 channel set up only with the Marantz prepro and is looking to upgrade to Atmos, with all of the extra channels that this requires. Also, it looks like the B&W fronts he has go down to 3 ohms, and their spec is that they'll take up to 1000 wpc.
That being the case, I really think that no receiver would do them justice. I'd concur that the Marantz 7706 would be the best option. The tricky part is getting the exact same amplification across the front 3 channels since the Parasound A21+ would then be on the left and right. Parasound doesn't make a monoblock that would match the A21+ for a center channel, so a new multi channel amp might have to be acquired. The A31 might be close enough, but in needing 9 or more channels at 250 wpc, that's going to be a chunk of change on top of the speakers needed.
It sounds like the OP has been running a 3 channel set up only with the Marantz prepro and is looking to upgrade to Atmos, with all of the extra channels that this requires. Also, it looks like the B&W fronts he has go down to 3 ohms, and their spec is that they'll take up to 1000 wpc.
That being the case, I really think that no receiver would do them justice. I'd concur that the Marantz 7706 would be the best option. The tricky part is getting the exact same amplification across the front 3 channels since the Parasound A21+ would then be on the left and right. Parasound doesn't make a monoblock that would match the A21+ for a center channel, so a new multi channel amp might have to be acquired. The A31 might be close enough, but in needing 9 or more channels at 250 wpc, that's going to be a chunk of change on top of the speakers needed.