Re-tubing my Audio Research Ref 110

Do the KT120 just drop in as a direct replacement for 6550c?

Did you contact ARC to confirm this and to find out what the KT120 should be biased to? 
 

Other than that, @jasonbourne52 provided the steps. Just have you source(s) powered off when biasing to be on the safe side. 

I would talk to to Audio Research, technical support. They are great. Personally I always replace with exact duplicate tubes… but that is me. I cherish the Audio Research sound and simply wish to perfectly maintain it. But, that is me. 

No need to call ARC. Yes, you can substitute the 120's for the 6550's*. 

And yes, as Audphile1 said, you want to disconnect the source (music should not be playing) when setting bias. You don't need to take all those steps the first guy who responded specified. To prevent any wildly off bias, wait ten minutes with the amp on and speakers connected but with no music playing and then do the first bias setting. And yes, your amp comes with one pot adjusting one tube with another "slaved" to that tube. So one bias pot for two tubes. The bias pot sets one tube to a specific setting and then the slaved tube is merely expected to be within a specified range. Don't sweat it. After that initial setting done to prevent any "flare ups" of potentially badly out of whack bias, just play music for an hour or two and kick back and THEN stop the source and check bias again resetting as necessary That is all you need. Not all that other stuff. After 20 hours or so you can check again. If your tubes won't bias to spec, you need to rotate the slaved pairs up to three times with different combinations and if they still won't bias to spec you got an unacceptably matched set (mismatched set). 

* KT120's are polarizing. They have grunt and extension but in the opinion of many lack the soul and midrange presence of good 6550's/KT88's. But you have already bought the KT120's so enjoy them. They are a good tube. Nothing to lose sleep over. 

Thanks all. And when connecting the multimeter Does it matter which lead goes to which point (yellow & black points on the ARC)?

Although not true of house wiring, on your amp black is ground and yellow is DC and likewise you will use your black Fluke probe to black and the Fluke red probe on yellow.