All of the units reviewed here I tested with a variety of active and passive bass guitars, and some of them I also used for synth or line-level audio. I test each unit under normal use as well as "worst case scenario" usage. In other words I will do things such as feeding them extreme EQ settings, playing very sloppily, plucking heavily on the low strings, using both very low and very high input levels, quickly alternating between staccato and legato, long rests and busy playing.
My primary testing rig for these reviews is a Focusrite ISA-One and Beyerdynamic DT990Pro headphones, with Canare/Neutrik patch cables. I also use other DI's and preamps for tone comparison or level matching, such as an Ashly LX308B, A-Designs REDDI, Countryman Type 85, Summit Audio TD-100, Phoenix (UK) "Nice DI", Radial JDI, Ampeg SVP-CL, and Hughes&Kettner Blues Master. I do sometimes listen through a speaker cab rather than headphones, but it is frankly harder to hear subtle differences that way.
It has been pointed out to me that the Beyerdynamic phones have a spike in frequency response around 20 KHz, so I have been hearing noise/hiss that many people will not hear through their speaker cabs. I consider that particular "inaccuracy" a good thing though, as it means that my descriptions of hiss will be relevant to recording engineers, and it also means that if I say a comp's noise is "moderate" or "not bad", that noise will probably not be noticeable at all under most real-world performance circumstances. Most of my reviews describe how noisy or quiet each compressor is, and I test and listen carefully for that aspect; but it's important to understand the causes of noise--different equipment, rooms, and wiring can cause a piece of gear to be noisy for one person and quiet for another. Please click here for more on that subject.
Beginning in 2012 I also use a spectrum analyzer (TrueRTA on PC with an Echo MIAMIDI interface) to test the frequency response across the entire range of settings of each compressor. So when I describe the loss or boost of any frequency range, for many of my reviews (2006-2012) I used only my ears; but for any newer review I am also balancing what I hear with objective testing. Interestingly, in most cases there is no significant difference between what I heard and what the computer tells me afterward.
I typically spend at least three hours, over the course of a few days, working with each compressor. Some units are easy to understand and master within that short period of time, but some others require a lot more exploration, study, and studio time before I feel I have understood their secrets well enough to review. There are some of these units which I have worked with for over a month before reviewing, and many more which I reviewed in a shorter time but where I had to go back later and edit the review as I learned new things about each unit.
I also have made it a point recently to start updating my older reviews when necessary and when possible. My oldest reviews were only one paragraph, and I knew so much less about compressors at the time that many important details were left out. Additionally I did not have so many points of reference, so I'd say things like "this pedal contains an actual tube!!!" or "it can also be used as a clean boost!", comments that don't actually mean as much as I thought they did. So when you see a review that looks too short and simplistic, that's why: it's old, and in need of an update. It's a slow process because I just can't afford to buy them all again in a short time; however that is what I have been trying to do, bit by bit--I have bought about twenty of the reviewed comps a second or even a third time. Many more remain to be done. If a particular review stands out to you as needing a makeover, feel free to email me about it.