As barrysandy said above... If you compare the 40 and 10 side-by-side, the circuits are identical down to every trace and part. The tube choices don't really matter much because you can easily put your favorite 6550 in the power supply and 6H30s in the audio circuit (I like NOS for both). The different output coupling caps could explain some of the sonic difference, but there may be other improvements brought about by changes in wire, connectors, circuit board material, or other things that are hard to see.
The Ref 40 and 10 are much more similar to each other than they are to the Ref 6 or it's predecessors due to the larger, more sophisticated, power supplies and complete dual-mono circuits throughout. The 40 and the 10 have separate transformers for each channel, separate sets of regulator tubes per channel and separate 6H30 tubes for each phase of the balanced signal. The Ref 6 shares transformers and power supply tubes between channels and shares 6H30 tubes between + and - phases of the balanced signal path. It also shares one chassis and one main board for both power supply and audio circuit.
I own a 40 and haven't heard the 10 in my own system to compare directly, but I did have a Ref 6 for a few weeks and it's a great sounding preamp. It's not as complete, 3-dimensional or effortless sounding as my 40 but it did offer a bit more transparency at high frequencies. My understanding is that the 10 sounds like the 40 with some additional transparency.
