mcintosh mc275 mk6 tube rolling ?

I have the same amp and I'm about to tube roll as well. one thing to note if you are using the balanced input the first small signal tube (closest to the input side) is not in the circuit. You still need a tube in there for the tube heater circuit so leave the factory one in there. That tube is for converting the single ended input to balanced. not used using the balanced input. 

as for what tubes have the most effect I've not go that far yet myself maybe someone else has. personally i tend to keep them all the same regardless when i swop them out. I personally like 6550 over the KT88 but that's personal taste. 

If you want to roll tubes, you should have a tester to make sure you are comparing good tubes. 

To see what's involved in making a proper tester see ieLogical Build Images

There are simpler that do less and more complex that are mostly beyond what a hobbyist listener requires.

The uTracer, a miniature Tube Curve Tracer / Tester. (dos4ever.com)

A low cost, easy to build diy valve/tube tester | Valve Heaven

A very simple emission tester for power valves | Valve Heaven

Good point about having a tester.   I found out the hard way that some of my “best” tubes were average at best.   
 

Since having a tester I sorted through about a dozen NOS e180f Mullards.  They were from a large batch and I have no doubt they were NOS but they all weren’t the same quality.   I matched them in pairs and put the ones with the strongest emmisions in my amp and it was much improved.  Great stereo image and a great mono image for lack of a better word with mono content.   
 

The original pair was the same , one was shy of min new and one was strong , much better than new so clearly mismatched.   
 

I think it is worth the few extra bucks to have a place like Tube Depot match and screen for low noise.  In fact, the tubes I have that are put on an amplitrex or similar matcher are exactly as advertised 

This is important with dual triodes like 12ax7 and 6922.    Often times the internal sections of the tube are not balanced.   It is much more practical to buy from a trusted source that matches than buy a tester for most people 

I only bought one because I started playing with rectifiers and antique tubes.  I didn’t want my amp or preamp to be the “tester “

This is important with dual triodes like 12ax7 and 6922.

If both triodes are used in parallel, the tubes can be matched using both triodes together.

It's really only a problem if each triode is used separately as in a phase splitter like a long tail pair. 2nd harmonic will increase if the gain is off, but some might prefer that as 'warmth'

A real benefit is testing tubes every few hundred hours to ensure match is still close and no tube is not heading south. Additional benefit is 'cleaning' the pins and sockets from the removal / insertion.