It's a good idea if new capacitors are in matched pairs also, as suggested by Frank Van Alstine many years ago about an ST-70 upgrades.
About the preamp, if it works well, sometimes a bad tube matching can generate an imbalanced sound.
In 12ax7 triode matching is important match mu and Ra. Mu is the gain and Ra (Mu/gm) affect directly output impedence.
if you have two tubes with this values:
even if the matching of the gain is almost perfect, the two Ra are very unbalanced between them.
The difference between the two values is about 10,000 ohms. This difference is a difference of 20% (50,000 out of 10,000 is a difference of 20%) and will negatively affect the output impedance of your preamp, creating an audible imbalance between the left and right channels of your system (in substance when you hear the music a speaker will sound louder than the other).
So is Very important when you buy Double Triodes for your preamplifier that they are matched for Mu and Ra. Max with a tolerance max respectively 10% and 5%.
Hope this helps.