How to calculate resistance for a sub woofer

First things first. Did you rebuild the power supply or not. IF you didn't STOP. Rebuild the power supply before you wipe out a transformer..

IF it's a fuse good if not... Hmmmm!! That ain't good on a 225.. Just to let you know. You HAVE TOO repair the power supply, if you don't BINGO! 200.00 at the most for all the caps and rebuild parts for the PS...Complete all new BB Cap replacement another 125 or so.

I have a 225 that is as PERFECT as any I've EVER seen...Truly museum quality.. Even the original RCA BP Mcintosh driver tubes, YUP..

Regards

Are you using 2 subs or just one?

My preference would be to drive the subs with line level from the preamp. That way, the sub is electrically isolated from the MC-225. If the PL-200 develops any issues, it can send a dead short to the amp. Not good.

Thank you all!

What millercarbon said.
Don' worry 'bout it.

I was using the 8 Ohm, but the amp died! Don’t know if this is related.

Is this sorted?
Connect this way:
Amplifier speaker out (- / + 8 ohm) > sub high-level inputs. (In parallel with your standard speaker connections.)

The PL200 is powered. That means it uses its own circuit that first drops your speaker level input voltage down to line level. It does this with a resistor of usually 20k to 50k ohms. What this means in plain English it does not matter what tap that sub is connected to the amp does not even "see" that as a load it is irrelevant. So you can connect the sub to any tap it won't matter at all and you can forget about the sub.   

Connect the Focals to either the 8 ohm tap or if you want to compare then connect to whichever one sounds best. Which one it is connected to does not matter either, it is just there as a convenient way of getting the best sound from that amp when used with different speakers.