Thanks for the link to the Bent Audio site. If I don't pry myself loose from this site I'll never get anything done today. That's beautiful stuff.
I'm with you on the need for high quality attenuators to preserve the best of the sound. My favorite 'preamp' is a custom made shunt/variable attenuator built on a huge Shallco rotary switch. The pass element is a Vishay bulk foil resistor and the switch position selects a single Dale resistor as the shunt for that volume setting. It has two switched inputs and 2 outputs. The sound is very clean and sounds better to me than any active preamp I have compared it to. Being a passive, it has serious functional limitations involving source and load impedance so it can't be used in every system configuration. In some configurations a powered preamp sounds noticably more dynamic if slightly less clear. I'll have to investigate the tapped transformer option. I know of one other company that makes them. The name Silver Rock comes to mind. They're costy compared to an Alps pot, but not if you're contemplating a high quality resistive attenuator like a Daven or a DACT.
I would be concerned about the bandwidth, maximum level and phase shift. That will have to be investigated. Great concept, though. When using magnetic products to pass line level signals, phase splitting and 'balanced' operation is an attractive option. There are benefits in reducing common mode noise, and presenting a 'split' signal to a Push Pull tube avoids having to engineer that part of the circuit. It allows the amp to be entirely symmetrical from input to speaker taps.
Regarding this preamp, I wish I had sought you out to chat about it. I came in the room multiple times to stare but you were engaged in conversation. It's quite an achievement. The cylindrical copper bits look like they were adapted from water pipe fittings. Am I right? I love the baroque mad scientist-y look. This piece is not for every system, and it should sit by itself for best aesthetic effect, but what a statement. Knowing the Foreplay circuit, it probably sounds pretty grand as well.
Is the copper clear coated or did you spend half a day polishing it for the show?