I'm not sure what you can tell from the closeup of the MC-12's audio board that tells you about build quality. To someone who has built and manufactured circuit boards, the Lexicon board is standard surface-mount, multi-layer design, which anyone who does conventional manufacturing processes would get. In other words, it doesn't speak to build quality. If you could get a copy of the layout, that would tell you lots more.
Here's what I appreciate about both pictures, the B&K and the Lexicon.
The B&K's biggest fault would appear to be doing voltage regulation far away, and off the board containing its op-amps and other ICs. See that board near the bottom, to the right of the transformer, with the rectifier, the 3 big purple electrolytic caps, and the giant bundle of wires coming out of it? There are two voltage regs which appear to set the voltages for the rest of the boards.
This is not good if you want good CMRR at the full-operating bandwidth of your ICs, especially the really fast ones. The local bypass structures might be OK on the B&K and may even mitigate this some, but clearly, the power regulation was built to a price point.
The Lexicon on the other hand follows standard good practices for highish-speed design, unlike most high-end audio boards I've seen. They use multi-layer, surface-mount design. Multi-layer lets you route closer and tighter as well as use continuous ground planes to control return currents and shield from noise. Surface mount reduces parasitics from component packaging: the capacitors are closer to pure capacitors than the leaded (ie. components with wire leads, not Pb) caps you often see on "high-end" designs.
These parasitics include inductance which slow down the response of the power supply bypass structures. Since many op-amps and DACs operate at many megahertz, and often much higher, it is essential to have a power supply bypass structure that responds at those frequencies. If you have a leaded bypass structure, you don't have that kind of response.
Also notable is the ferrite bead on the far left of the close-up picture of the Lex. The picture doesn't show the whole power regulation scheme, but having looked inside MC-12s, it is done with more care and attention to detail than most other power regulators I've seen. That ferrite bead is one indicator of what they did.
Having said that, one poor example of high-speed design is Theta's Extreme DAC board. Note all of the big yellow, bulbous caps on the DAC board (the board to the left of the board with the blue resistors). I would not be surprised if those were 2-layer boards. Considering how cheap 4-layer, nevermind 8-layer, and if you were charging high-end audio premiums, HDI boards, are, to me, this is one of the most inexcusable practices of high-end audio. The circuit layout expertise is like from the 70s. Based on this alone, the Lex already has much higher build quality.
The Theta's power regulators also have to work through that slow modular connection to the DACs. It's like B&K-style design!
And finally, I LOLed at the pink heatshrink on the two voltage regs near the bus connector that keeps it from shorting out pins. If you look at the Premium card in the same gallery
, they appeared to have fixed it by flipping the regs around. Build quality much?