Can I Pair Audio Reseach Ref 3 w/ the McIntosh MC2505 Poweramp?

Whatever you buy, you should consider the scenario if/when you upgrade your amp. I have an MC2200 McIntosh amp which may or not share the same sonic qualities as your amp and I tried it with a couple of different preamp's - mostly whatever my friends were willing to bring over and try and including my own SS NAD preamp, and the best sound was from an Audible Illusions 3A. We tried it with one of my friend's ARC LS17se and for some reason we couldn't get the volume high enough for a decently loud playback. Not sure why, probably gain mismatch. None of this will probably help with your decision since my speakers are completely different from yours- ML Aerius, and I have a huge listening room, but use the data points for whatever they're worth. All in all, there are a lot of folks out there, particularly on the McIntosh forums, that claim MAC stuff sing best when mated with other MAC components.

Yes I've heard this about Mac gear, that it stands out most when paired with other Mac gear. I hope this is true for vintage and modern combinations of Mac gear as well. But matching gain levels is another factor I hadn't considered, but now I will. I believe the c2300/ 2500 has a function that will match gain fairly accurately, someone please confirm this.

I was also considering biamping the 2505 with another 2505, and using the c2300/2500 to feed signal to both power amps (provided I decide on buying a Mac Preamp). In that scenario the pair of MC 2505's will provide over 120 WPC to each speaker.

I have had McIntosh equipment, ARC equipment and a number of combinations of the 2. My conclusion has always been that an ARC preamp and Mac amp or vice versa delivers the worst of both worlds. ARC/ARC is my personal favorite. Mac/Mac my second favorite. Far worse is any combo of the 2.

I was also considering biamping the 2505 with another 2505, and using the c2300/2500 to feed signal to both power amps

Why would you take the weakest link in your system and add another 2505?  The 2505 was Mac's first solid state design, and they have learned a lot since it's introduction.  In addition, IIRC, you cannot bridge a 2505 to run in mono, you need to run each amp in mono parallel which only increases current and not voltage, therefore, you system will not play louder than your stereo setup.