ARC DS450. Should match well with your REF 3 and should have plenty of power to drive the Strads.
0461...the impedance mismatch is on paper, meaning given the nnumbers, it suggests they may not be suitable for each other. The best thing to do is to try to listen to the pairing together. If you have any McIntosh dealers around you, if they do not carry ARC products as well, I would take your Ref 3 to their store and listen to it with a MC501 or MC1.2KW and test them out together. If they sound good, then no problems. I was just making an observation based on the reported numbers.
0461...that's great that you have narrowed it down to the MC501 and MC1.2KW but unfortunately they will both not fit that well with your ARC Ref 3. There is a potential impedance mismatch and the place where it will be most apparent is in the bass fequencies, where the Ref 3 has an output impedance above a 1000 ohms. The input impedance of the MC1.2KW is 10Kohms (which gives you less than the minimum 10:1 and certainly less than the often desired 15:1 or 20:1 ratio between the amp's input impedance to the pre's output impedance. The Ref 3's overall output impedance of 600ohms balanced and 300ohms through single-ended connections will barely get you there in terms of the desired ratios but as I said in the important bass region the Ref 3's output impedance goes above 1000ohms. This will cause attenuated and loose bass performance most likely and will therefore muddy up the overall resolution and thicken the sound.
If you look at ARC's power amps (both SS and tube) they are all above 100K ohms in terms of input impedance. So you may have to find a low output impedance preamp to match with your McIntosh SS amps, maybe 1 of their own preamps like the C500T.