cv wrote:That upper rolloff in the OPT comes from a number of distributed capacitances and leakage inductances in the transformer - it will not be a simple filter. Lots of transformers have weird resonances up at the top of their range. In fact, some makers rely on a resonance to push up the bandwidth. Otherwise, I vaguely recall that a well behaved transformer has a second order rolloff up top.Ie it's a very complex low pass filter with less predictable phsae shift. That's one guess for why the channels are not summing as you'd expect.I would think that relying on the OPT rolloff would be detrimental to sound. The load presented to the tube up there is going to be extremely reactive and screwy.