About tube (valve) compressors:
For people seeking fat, warm, "vintage" tones, the idea of a tube compressor is very compelling. After all, tubes and compressors on their own are each advertised as making everything sound fat and warm, so the two of them together must be amazing; and The Beatles and most other famous bands of that era were recorded with tube compression, so let's go buy a tube compressor, right?
Unfortunately, it's not so easy. There are several issues:
• Most so-called tube comps on the market are really just a solid-state optical comp followed by a tube make-up gain stage. So really they are the same as using any optical comp fed into any single-tube preamp. This can sometimes sound good or even fantastic, but let's not mistake what we are actually getting. Nearly always, in modern products, the tube has nothing to do with the compression part of the device. So if you have an optical comp and either a hybrid amp or a tube gain pedal, you've already got the same thing as what's typically sold as a "tube compressor".
• The legendary studio devices used in vintage recordings had a lot more going on inside them than that: multiple stages of different kinds of tubes, not just a single 12AU7; and audio transformers and opto cells that had their own tone and action that is hard to replicate using off-the-shelf components. A single preamp tube can never sound entirely like a more complex all-tube circuit with input and output transformers. Again it may sound great, depending on the particular device and your personal tastes, but one tube is only a small part of the overall tone picture of a vintage recording. You may even be better off using a digital or solid state tone shaper that emulates more of the sound qualities of the whole device or system, rather than relying on just a single component like a preamp tube.
• With tubes, voltage and current make a big difference in how the device will sound. Electronic designers have to carefully choose how the tube will be powered in their circuit, to optimize its performance; but all too often the Accounting Department makes them choose the powering system that is cheapest, not the one that will sound best. Cheaper usually means lower voltage, lower current, and DC instead of AC. It is possible to make a tube device that sounds good with low voltage DC power, but that is an extra design effort that many manufacturers do not bother with. They seem to reckon that if the tube produces gain at all, that's good enough.
• Realistically, there are many tube-containing devices out there where the tube does nothing good to your sound. They can make your tone mushy and indistinct, or they may do nothing audible at all. Again this is about marketing and accounting: they want to sell you the idea of "fat, vintage warmth" without actually providing anything but a weak, poorly-executed gain stage. NOTE: This is in no way a criticism of all tube devices, just a warning about some of them, especially at the lower price points.
• Many tube devices are vulnerable to picking up ambient noise from household wiring, nearby power transformers, flourescent lights, ground loops, and so on. Noise problems like this can be a real pain to try to solve. That's the one thing that keeps me from using a tube comp very often--I can't stand hiss, and I get frustrated by trying to hunt for mysterious noise sources. I've gotten multiple emails from people saying their tube comp is not noisy; I suspect instead it was their electro-magnetic environment which was not noisy. Sometimes it's even due to a differently-designed grounding system inside your amp or another processor, something that cannot be easily changed. The tube comps that I have reviewed positively happen to be ones that showed some resistance to outside noise.
So what's the answer? Honestly, all you can do is use your ears, and don't believe the hype. Don't believe the advertising text about fat vintage tone, and don't fool yourself into thinking any device that has the right specs/features "on paper" will actually sound the way you hope. Try before you buy, or buy from a place that has a good returns policy, whenever possible.
All that said, there are some great-sounding tube comps out there. My favorites are the Effectrode PC-2A, Retrospec Squeezebox, Lightning Boy OP-2, and Markbass Compressore, they're amazing. The ART Levelar is not too bad for its low price. And note that with a rackmount tube comp you are more likely to get a properly-designed circuit with appropriate power, compared to most pedals. I just haven't covered the rack units so much because they tend to be large or expensive--I will try to provide more reviews and information on those in the future.