I have watched and witnessed the "matched pair" tube phenomenom over the years with a bit of dismay as it pertains to Macs. Matched pairs are useful in many other amplifiers however it is a waste of dollars for McIntosh tubes amps. Why? Because the Owners Manual says so, I'll quote it here from the MC-275 manual page 8 "Important: ...... The patented McIntosh circuit delivers its advertised specifications without any need for these controls (bias) and is not dependent on carefully balanced tubes for its performance. With McIntosh you can install the amplifier and forget it." This is applicable to all MC series tube amps.

Why use matched pairs? Basically you want them to be biased equally for a balanced output in the plate circuit. You want each half of the circuit to contribute exactly 50%, no more - no less for the lowest distortion. Push-pull is the most common circuit for output stages and common sense tells you why it's a good idea in this application. So you have 3 choices: (1) provide a bias adjustment [like the old Dynakit Stereo 70's did], (2) plug in matched tubes [still not a perfect solution unless the bias circuit is perfectly balanced] or (3) design a magic circuit that does it for you. McIntosh's proprietary Unity Coupled circuit cross connects the ouputs to the cathode bias circuitry of the opposing stage keeping both halves in balance dynamically. This precludes the need for bias controls or tube matching.

Still, it certainly won't cause any harm or degradation if you do. But you are spending $$ you don't need to and you will not see (on test equipment) or hear any improvement.