Everyone has a different idea of what clean tones are ideal for them, how much hair they like when they dig in, etc. You really have to play through the amps you're interested in, after listening to clips and getting recommendations, because I think especially when it comes to clean tones, your personal instrument, the way you play re: your touch, and of course what you imagine to be great clean tones matter a great deal.
Nonetheless, I'll dive in. I've broken some amps down into the categories I think about when I listen to an amp; all of these are amps I've personally played through, as I don't believe in talking about amps I have no experience with - and there are plenty of amps I've never played through, so take my advice with a grain of salt.
1. Single Channel amps that give me pretty clean tones when I use the guitar's volume control:
If you're a player who likes to use the volume control on the guitar to control the gain on the amp (I am) I have had great luck with cleans on a PRS DG30 and even the HXDA, which gives me those "Wind Cries Mary" Marshallesque cleans when I turn down the guitar volume. I think most people wouldn't think of these as clean amps, but with the guitar volume control, they are. However, maybe you're not a volume control player, and I'll admit I like a tiny bit of hair on clean tones when I dig in.
2. Amps with a broader frequency response:
If you like a fuller frequency response in your clean sounds, there are some very nice Mesas on the market; I particularly like my Fillmore and the California Tweed is also great. I agree with the poster who liked the Magnatone, I did as well when I played one. It's different from a Fender tone, actually in some ways full frequency like the Mesas, only...with a different emphasis and more transparency. I use my Mesa Lone Star 100 for clean sounds as well; in fact, turning the mids down and the treble up approaches that scooped Twin thing. The amp has a ton of headroom, and I think the clean tones are still very competitive.
In a different way, my Two-Rocks had fabulous, transparent, gorgeous clean tones, and I think of them as full range clean tones, i.e., they still have bass, mids and treble in relative balance. A lot of folks feel the Two-Rock clean epitomizes the 'boutique clean tone'.
3. Amps with a more 'tailored to taste' frequency response:
There's also the classic scooped Fender Blackface Twin sound. I'm a fan of these amps, if you can find a good one (by that I mean, vintage '60s amps, or at least handwired versions). This was an amp without a ton of bass, the midrange is recessed or scooped, the highs are predominant. These have a legion of fans who regard them as the ultimate clean tone. I'm not a fan of most of the new ones, but the old ones really are fantastic.
Vox amps filter a bit of the bass, goose the high frequencies (hence the need for top cut), and have a characteristic 'kerrangggg' in the mids. And classic Vox tones are hard to beat if you like a sound with less bass and a little bit of that 'crushed glass' thing when you crank it up.
Perhaps the finest Vox style tones on the market are the Matchlesses and Bad Cats with preamp sections that are based on the Vox design. They're simply better built and they can be adjusted to do more things.
Also take a look at the Divided by 13 models, Dr. Z, Victoria, and Clark for a more boutique taste of these vintage designs. These are some of the nicest vintage style clean tones on the market today.
Hope this helps!
