A lot (but not all) of the "high-end" boutique gear is built by hand, and as such, they're generally not mass-produced, nor is there usually much of a budget for having an outside company do surface mounting, wav-soldering, etc. Part of their high-end price tag (in addition to the TLC and quality components that go into them) is the hand-made (LABOR) aspect of the device.
But that alone doesn't make them any better sounding. There's hundreds of other reasons why something sounds better or worse than other gear.
There's a lot to be said for SMD in terms of keeping signal path short, keeping hum and extraneous noises out of the circuitry, and so on. (One big, through-soldered cap doesn't make a product sound any better than a similar unit with the same spec cap surface mounted.) Putting everything inside of one RF-Shielded box, all on the same Circuit board, with the same highly regulated power supply can do wonders for overall specs and performance.
As many have already commented here very eloquently, it's a lot of complex factors that contribute to a device's performance. How a vintage U87 sounds compared to a modern U87 is certainly worth discussing, but it's more than just SMDs vs. discrete parts. The same can be said for the early AKG 414's vs. what they sound like now. It's a long list, with a lot of culprits.
I use Grace preamps as well as RME and Mackie Onyx preamps, depending on the job and the project. As far as I'm concerned, the sound - or lack thereof - has NOTHING to do with whether or not my gear has SMD parts inside.
Joe Hannigan, Producer
WestonSound.com - Philadelphia, PA & Greenville, DE
Acoustic Music Forum co-moderator.