I think the weakest link in your system is the source. Nothing personal, but I've never been a fan of Marantz. I find them too warm, laid back, and slow. To my ears, their top end is a little too soft, and their bass is a bit flabby. Everyone has their opinions though.
Why do I mention my preceived Marantz downfalls? Because I believe a BDA-1 will bring the music closer, be more transparent, and become quicker and tighter. If these are the changes you're looking for, the BDA-1 is definitely it.
I've also come to the conclusion that pre-amps are probably the most over looked and underappreciated components in most systems. In auditioning a bunch of different systems before getting what I presently own, I was very surprised by the differences pre-amps made, especially ones with outboard power supplies. In pre-amps, power supply is vital. With a better power supply, music becomes cleaner. The system has better resolution and detail. Its amazing how much grunge gets into a pre-amp. Adding a DAC to a pre-amp is very convenient, but potentially adds to the noise in the system. All just my very unscientific opinions gathered by listening. I'm sure someone will disagree.
So to sum it all up, IMO the BDA-1 will bring the most immediate difference, followed by a BP26 down the road. There's a very good difference between the BP25 and BP26, but nearly as much of a difference as between your Marantz CDP and the BDA-1.