Reading all of your posts to this thread, again this morning, it has struck me that you've never expressed why you've chosen to insert the Adcom GFP-555 pre-amp between the Emotiva XDA-1 (USB DAC) and the various amps you'll be testing, but I can see from you latest post, that you're adjusting the pre-amp's gain to regulate the signal strength coming into the Emotiva Mini-X.
It turns out that when I'm using my LCD-2 with the Mini-X, with my DAC providing a 2V rms signal directly to the Mini-X RCA inputs (without a pre-amp), I measure a SPL of 85 dB between the ear pads with the Mini-X volume control at 9 o'clock. I don't know what SPL you're achieving at the LCD-3 ear pads when your Adcom pre-amp is at 12-o'clock and the Mini-X is at 9-o'clock, but I'm guessing it would be somewhere around 85 dB, given that you find those gain settings to provide a reasonable auditioning volume.
Making a lot of assumptions in above paragraph as to output voltage of your DAC (being the same as my DAC), efficiency of the LCD-3 (being the same as my LCD-2 rev.1), and SPL at the ear pads (being the same as I measure with the JL Audio iPhone app and the lapel mic I'm using)... I can nevertheless propose that when your Adcom gain is set at 12-o'clock, you might actually be at or near unity gain - where the Vrms leaving the pre-amp is the same as the Vrms coming in from the DAC. Can you confirm this through documentation or measurement?
I ask because I'm concerned about the impact that your Adcom is going to have on all of your impressions, even if you were to leave its gain fixed as you compare one amp to another, due to injection of the Adcom's deviations from perfect neutrality and transparency (no matter how subtle). But I'm also very concerned that if, for any reason, you vary the signal strength coming out of the Adcom as you compare amps, you could significantly alter the dynamics or weight of what's heard at the headphones vs. setting the Adcom at unity gain.
In the Owner's Manual of the Decware ZSTAGE (single-tube triode gain stage) Steve Deckert writes that one can reduce the dynamics and weight of a DAC/amp combo, by inserting the ZSTAGE between them and adjusting the ZSTAGE gain to something less than unity gain, while simultaneously increasing the gain at the amp to maintain a constant SPL at the headphones or speakers.
Conversely, he writes that the ZSTAGE (which is really just a tube pre-amp with a single pair of RCA inputs and outputs), can be used to increase the the Vrms going into the amp (up to about 5 Vrms, depending on the tube you've selected). When the amp sees a stronger signal than what the DAC would provide directly (greater than 2Vrms, for example), dynamics and weight can be increased, while maintaining a constant SPL at the headphones by simultaneously decreasing the amp's gain.
I can attest that I've experienced this ability to customize dynamics, first hand, using the ZSTAGE to either increase or decrease the gain seen by the amp (above or below the 2Vrms provided by my DACmini CX DAC section). And last night, just by inserting a pair of Harrison Labs 6dB inline attenuators between the DACmini CX and Emotiva Mini-X (reducing the incoming signal from 2Vrms to 1Vrms), I could hear a reduction in dynamics, after volume matching.
So, with tremendous respect, I'd like to propose that you remove the Adcom from your test environment when using amps that have their own volume control - opting instead to compare these amps with a direct signal from the XDA-1 DAC. This could make volume adjustments tricky for the more powerful amps, but it would rule out any sonic traits imparted by the Adcom even if it were fixed at unity gain, and would also rule out the impact on dynamics that can be imposed with variations to either side of unity gain.
What do you think?
Edit: When Steve Deckert uses the word "weight," I've concluded he is referring to bass extension and bass energy. I'd also like to add that in e-mail exchanges with Jan Meier (of Meier-Audio), he has agreed that an increase or decrease in dynamics can be affected by increasing or decreasing the amp's input signal strength, but that the degree to which this is possible varys from one amp to the next. Some amps are not as susceptible as others, but he didn't explain why.
Edited by zilch0md - 6/25/13 at 8:04am