Matt, I'm gonna play devil's advocate!
You seem to be drawn to the Warm Audio TB12. Let's compare the 500 series. $549 @ sweetwater. The API 512C used $695 @ Vintage Audio. $146 difference.
The TB12 tries to emulate the API in some respects, while offering extras like nickel transformers, various caps, to give a lot of tweakability and versatility in tone. Sounds good so far. Imagine having all those options in an actual API preamp, the ability to swap out components and totally detune the circuits. Do you have anything better or just different tones that are a total mess?
http://www.soundonsound.com/reviews/war ... tone-beast
~ Both its Gain & Saturation and Tone Control sections can transform the preamp into a saturation generator, with a sonic scope ranging from extremely subtle coloration to the wildest fuzz sounds. Really, fuzz in a preamp? Stomp box okay, maybe before or after a fine preamp??
~ Usually, this kind of technology can only be found in units costing two or three times as much as the TB12. Always a red flag!
~But while the TB12 has all the ingredients typically associated with such 'colour', such as the transformers, Tantalum caps and discrete op-amps, please don't think that using any of these options will drastically change the sound in any given situation. In many use cases these variations remain very subtle, and will only be audible to the critical listener.
~capable of turning this microphone preamp into a hefty distortion box. Again, in a mic preamp??
https://www.gearslutz.com/board/reviews ... -tb12.html
~I found the Tonebeast to be quite versatile indeed, but to also definitely have its own character. At more moderate settings I could dial it in to where I found it compared favorably with my favorite vintage API 312, although it was not quite as dynamic, had a bit more mids, and a little bit leaner lows. When put up against a vintage 1073, the Warm was again leaner in the lows, but also made the 1073 sound a little dark, missing just a little bit of the “cut” of the TB12. I also felt the Tonebeast was a smidge less 3D than some of the much more expensive preamps I put it up against, although I never felt it to sound like it was outclassed, even though some preamps were more than six times as expensive.
If you are looking for a stomp box, API Tranzformer GT for guitar or there's one for bass
http://www.sweetwater.com/store/detail/TranZformerGT
https://youtu.be/DXKo72_doyo
I think the last review is particularly telling, because sometimes it's the tiniest of details that really make the biggest difference,(it's usually the difference between semi pro and pro) and especially when it's rounded off with a "preamps more than six times as expensive" (the red flag). It seems the excitement about the Tonebeast is it's tonal variation, the ability to sound like a stomp box!
The thing about the API stomp boxes, is that, " the TranZformer GT combines an API 525 feedback-type compressor with a guitar-optimized 3 band equalizer, both independently in/out switchable. The unit also features phase invert switches on each output, ground lift, clip indicator and transformer output." You don't get an eq or compressor, certainly not API ones in the Tonebeast. Going the little extra as a bang-for-the-buck kinda guy, I also search for the values in what I can actually buy something and make something work, like used when it's advantageous, or sale prices in consideration.
So the $146 difference between the Tonebeast and a genuine API, is a no brainer for me. Yes I am adding the extra for the powered rack, figuring at some point you will want to add more components, and offsetting the dreadful power supply in the non-500 series version. And in the future, when you decide to add another component in the 500 series, you will be glad that because of the rack purchase, better and cheaper possibilities of totally pro gear are available to you.
~Devil's Advocate