Update on a regulated air supply.
More to report than I expected. I thought the results would be a matter of nuance, not a matter of dramatic improvement, but I was wrong.
I had been using a Rena 400 aquarium pump with two surge tanks, of 4 and 20 litres, with 1/8 tubing. The sound was very good indeed.
Then I moved to a HiBlow 40, a much more powerful pump. The first surge tank is a LEAKY 4 litre plastic bottle filled with cotton swabs, plumbed with 1/2" tubing and connected to a Fairchild precision regulator. From there 1/4" tubing connects to the two previous surge tanks.
The Terminator (lord, how I hate that name) runs sweetly at 9 mm of mercury (or 9/760 = 0.012 atmospheres), and has an interesting set of tracking characteristics. My Koetsu tracks the highly challenging bass tremolo on Bells of St Anne de Beaupre (Real Time Records "Power and Glory"), but not many of the soprano arias. At 20 mm of Hg, it tracks the sopranos flawlessly but not Bells.
The quality of sound improves dramatically from Rena to HiBlow. It is smoother, less artificial, more focussed, and more elegant. Improvement in focus continues, albeit to a lesser extent, with more pressure up to 20 mm.
All reports preliminary, you understand. I just set this up, and so may have to revise everything I just said. If you want to try it, let me know what you think - it's small change for a real improvement, IMO. What's more, the physics predict it, so I suspect that the improvement is real.