I can't keep up with you! My Sennhesier HD600 is due to arrive today and now you're singing about the HE-500!
Note to self: Remove FlySweep from #27 on my list of 100 people I will allow to live at the end of the world. Moo-haha!
No, from what I've read elsewhere, you're giving the HE-500 exactly what they need, but that's what I love about the LCD-2, when it's given a neutral and transparent signal, it adds exactly the colors I want, where I want them.
I know well the advantages of a tube pre-amp, and although I found it undesirable into the MG3 to LCD-2, I have no doubt it would be just the ticket with the somewhat dry HE-500.
I just yesterday shipped out a Decware ZSTAGE (triode pre-amp) that I sold via the Decware forum. There's a twelve-week wait for a new version from Steve Deckert, so I didn't have much trouble selling it used.
With a choice of five 12AU7 and one 12AT7 (three of them NOS), I was forever rolling, but I can say this with certainty - none of those tubes offered enough transparency to suit my affections. When used with the Emotiva Mini-X a-100, the ZSTAGE did a wonderful job of taming the too bright and harsh treble - into my LCD-2, which is known to have shelved highs, in the first place (that's how bad the Mini-X was.)
But these improvements came with a loss of transparency, some tubes being worse than others, but none of them lacked an unmistakable, yet faint, resolution-degrading "veil." And I'm not talking about the even order harmonics one can "enjoy" when clipping with a tube (where the grid current increases, causing a "desirable" distortion as the wave forms are clipped asymmetrically when overdriven vs. the undesirable symmetrical clipping one gets when op-amps or discrete component solid state amps are clipped.)
Well short of clipping, I, personally, have never heard a tube that wasn't adding at least a little bit of distortion, all the time. Keep in mind that my only experience with tubes is that had with the Decware ZSTAGE, but I find my observations to be consistent with a consensus of more experienced Head-Fi and HiFi enthusiasts.
Without question, every tube was a sonic adventure, injecting their individual personalities into the signal, with each of them offering varying degrees of warmth and color that were very appealing, but all with that slight degradation of transparency. And keep in mind that the LCD-2 is not known for being a highly resolving headphone.
Prior to selling my Beyerdynamics T1 - for which I had never found a proper amp (the Burson Soloist came close) - the ZSTAGE, when equipped with a made-for-3M-Wollensak, 1956 NOS Mullard 12AT7 did wonders for the T1, but I had to apply 6 dB inline attenuators at the ZSTAGE inputs because the 12AT7 is a "high mu" triode, offering about twice the gain of the 12AU7's. This worked very well at giving the T1 a signature I liked, but... it threw a fine gauze over the T1's spectacular resolution. Arrrgh! Everything's a compromise!
Ironically, given the T1's resolving power, a majority of folks in the T1 fan club recommend tube amps for the T1. So, somewhere, there must be a tube or two that are truly transparent, but I haven't heard them.
Lastly, the rolling of tubes drives me crazy, anyway - as does the rolling of op-amps! And by the way, I'm such a transparency hound, that I run my iBasso PB2 portable amp with dummy buffers - they're just shunts (nothing active in the output gain stage), using only LME49990s in the input voltage gain stage, because I have YET to find a sufficiently transparent op-amp for use as a buffer (the HA5002 comes close, however.)
I love me some transparency!
And THAT is what I am getting with the Resonessence Concero > MG3 (made especially evident by my highly-resolving Definitive Technology SM45 near-field monitors).