I purchased the Whitebox 4x12 cabs loaded with Celestion G12 -K100 speakers recently and wanted to wait a few days before giving my take on them.
First, the cabs arrived in flawless condition. I really couldn't think of a complaint despite my trying. On opening I was very impressed with the build quality. It is very solid top to bottom. Tolex looks nice and I was also happy with the front grill.
Now before I go into the sound, I want to say I am using a Blackstar HT-100. I bring this up because I'm sure amp choices can cause particular biases in tonal opinion. Before the Whitebox, I was on Mesa 4x12s loaded with Celestion Vintage 30s and for me... the Mesa just didn't match right with the Blackstar. Initially, I blamed the amp and even almost sold it. However when I plugged into the Whitebox, the sound changed dramatically.
First, these cabs + the G12-K100s give a more flat frequency response. For me, this meant a more wide open sound, something I needed with the already mid heavy Blackstar. With Mesa cabs, I just wasn't getting much low end and the mid range push from the V30s caused some harsh frequencies to stick out at louder volumes. With the Whitebox, I got the low end I needed and the harsh sections of the frequency response had smoothed out.
I'm trying to avoid abstract terms like "my amp came alive" or "it's the coolest ever!" but I will just say, for me, these are my favorite cabs I have ever used.
I am a Whitebox convert and just don't see myself switching to anything else.