Can you hear it ? A little more noise and distortion ?
@inna Yes. No tube or transistor is perfectly matched. Most 'matched' power tubes are only matched at one operating point, usually on a tube tester. To really match them properly you have to use a device called a curve tracer, which can display the linearity curves that result as the tube is subjected to a variety of operating points. The first is a 'static' match and the latter a 'dynamic' match.
And then the tubes get installed and over time they drift. Its rare that a pair goes downhill and stay matched. So distortion goes up, output power goes down and output impedance goes up. You can hear it!