I concur with @almarg, @hilde45, and @zavato… Control of independent variables is of the utmost importance in the review process, and should be implemented as much as possible to achieve reasonably meaningful findings.
In a perfect world, twin copies of the target component or cable should be used for periodic comparisons of performance throughout the break-in process… One copy being the full break-in target, and the second one as a control with “low mileage”. Now suddenly, I hit myself, because I just realized that I had a perfect opportunity to do that when I examined the Rowland M535 bridged a spell ago… I should have started in stereo mode, and used one of the two units as a low mileage control, instead of breaking-in the pair as a bridged set. Oh well, next time I evaluate a bridgeable amp, I’ll apply this technique for sure.
Would be nice to track voltages, air temperature, and humidity… Next time I am born I’ll make sure I stay fully sighted, so I can read measuring equipment… Oh well *Grins!*
On the other hand, I do control the test environment as much as possible, as follows:
I maintain system configuration to be invariant during each individual evaluation phase. This means that all components remain the same; cabling remains the same; usage of AC outlets remains the same; no equipment has moved around stands; layout of cabling on the floor remains the same; no furniture has been moved, orientation of window treatments remains the same.
All ancillary equipment is already well stabilized: In my case, all equipment has been with me between two years (cabling) and 14 years (CD transport)… I was forgetting equipment support benches (60 years).
Break-in process continues 24/7, except for power-off time during thunderstorms and for discharging capacitors (did this twice). Did any critical listening at least several hours after the last power-up… More typically, days or weeks after.
I make consistent use of review material… A test CD contains the same sampling of music tracks that I have used for evaluating equipment at home, at shows, and at stores for the last 15 years… In addition, I use several other CDs representative of music genres of interest to me.
While I listen to entire CDs during critical listening, I do concentrate on particular passages that I have known to expose possible flaws or merits in a review target: harshness from intermodulative artifacts, pillowing/unspecific bass, harmonic exposure changes in broad treble to bass arpeggios, transient clarity vs opaqueness, decay complexity, staging/imaging changes, very low level information, ambient noises, performers’ subvocalizations.
I document observations in contemporaneous notes also logging dates and break-in hours, which when cleaned up form the basis for diary posts, and in an ancient past, I have integrated into published reviews.
Use a break-in tracker spreadsheet… This maintains break-in status for each day, hours of operation each day, start time, power down time, total hours count since beginning of project, and completion date projections.
At the end of each project phase, for instance use of an integrated as a complete integrated, I make minimal changes to start the next phase, which might be for example feeding the linelevel signal from the linestage of the integrated into my reference monoblocks… I will use for this the same pair of well broken in XLR ICs that I have been using for the last two years from my reference DAC to the monos.
I will run this configuration for at least a couple days before any new critical listening, and will use the same tracks and passages that I used on the integrated. I will probably need to go back and forth between full integrated and its linestage subsystem into the monos to derive a reasonable assessment of the difference. Yes I know, the reintroduction of an IC will somewhat smear the results.
* I would then use the same XLR ICS when I test the output of my reference DAC into the integrated linestage + amplification subsystem.
As you might imagine, I can’t examine dozens or even a handful of components a year this way: It is a very time-consuming process. Never the less, it is for me a happy labor of love which I enjoy sharing with fellow lovers of music and sound… Others may feel otherwise.
Regards, G.