Probably a little rolled-off on the treble.... Frequency Response: 2Hz-200KHz, -3dB I'm sure -3dB at 200kHz is audible to you, but it ain't to me. My hearing must be going! I think a lot of people are used to hearing false treble boost and distortion and have acustomed themselves to thinking that's normal, then when they hear truly flat response and no audible distortion up high, they perceive it as rolled off. Just my 2¢ worth.
The Melos sounds similar on the treble. An "apparent" slight roll-off. The Apex Peak (depends upon tube) doesn't have the roll-off, yet is about two leagues higher on the transparency and lack-of-distortion scale than the Melos.
Also I respect what you are saying in regards to "ultimate resolution and transparency" however can you quantify what this means in relative terms? Are you talking 95% of resolution and transparency of a $1200 amp? Are you sure you're not subject to some psycho acoustic phenomenon?
These things are subjective and to a large part dependent upon the source. I don't know if the following helps. I hate doing stuff like this BTW -things could be slightly out-of-order or someone might just think I'm F'ed in the head so take with a grain of salt. But here is where I see things on the transparency scale:
Headroom Bithead < Headroom Microamp < Headroom Desktop amp / Lyr (JJ tubes) / M-Stage / M3 (OK op-amp) <= M3 ($$ op-amp) / Melos (stock) / Woo 6 (stock) < Melos (modified) / Woo 6SE << β22 / Peak (w/ good tube) / Frank Cooter's amps.
Note the minor differences among the comparative operators < <= <<