Great initial question! I was wondering about this...How about this, I'll try it, and post the results?
Here we go: tlm103
into an ART MPA Gold with Siemens und Halske Valvo NOS tube. First read with the shield, second read without the shield. I'm a voice over guy, so to accomodate you rock gurus I read a sentence from a technical program and then sing it really badly with whack dynamics - blustery, high and screamo, and then up close and growly. Enjoy. http://www.lanceblair.net/files/ARTMPAShieldTest.wav
BTW, the MPA settings are High Voltage, Input 1 o'clock, Impedance 3 o'clock, and Output 2 o'clock.
Now I need to get a green sharpie, although my clients say they loved my upgrade from red to black.