I received my CD7 back from Audio Research yesterday. I listened to it last night, and then again this afternoon. Total "break in time" about 5 hours.
The difference from before the mod till now is:
Air. The sound stage and music are riding on air. On top of this when an instrument is highlighted in a band (snare drum, trumpet), or vocal piece (soloist) its like the performer steps up. I have not heard that quality before.
Also the level of micro detail is increased by quite a margin. In particular in the midband, and high frequencies.
I now truly believe Leonard was accurate with his statement that the sonic level improvement was large. Though I must say, overall it sounds like the midrange is boosted up slightly. Before I thought maybe the bass was too much.
My associated equipment is Ref2, D400 Mk2, Wilson watt/puppy 7.
The new tube; 5881 tube is installed horizontally, slightly forward, and to the right cabinet edge of the old 2 power regulation tubes. The older power regulation tube holders are "capped off".
Audio Research also ensured my unit was fully up to spec (turn on mod), greased, lubed the transport.
Again another very satisfying experience from Audio Research.
I am not a dealer, not in the audio business, nor associated with Audio Research in any way.