Best Headphone Preamp Cost No Object

Thank you, all of you so much, for your great responses. As I had hoped, the units that you listed were among the ones I was already becoming familiar with. My personal choice up to now has been the Headroom MAX with stepped attenuators but I didn't want to overlook any good competition to the MAX and regret it later.
I like the Cary too from what I've read but between the high initial price and the overkill of having a versatile piece of equipment that I would only use for headphones makes it even more expensive.
I've also read about the listening experiences of several Headroom product owners that depict the overall "Headroom" experience as being clinical, cold, transistory? I believe these were various models of older units (2002 or earlier).

I'll make my move in a month or two. If anyone else reading this thread has a great unit to share please jump in.

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I own & I am extremely happy with my latest-version Headroom Max (with HD650s with a Zu Mobius cable); Headroom's new version has a new "reference module" that
is stunning, a significant improvement over the original Max. It is built as solid as a rock, generates no heat so you can leave it on all the time, and has a small footprint, & takes no care & feeding like tube amps. Even if you have a tube amp, I would have a Max as a backup. It's one of the great bargains in high-end. The midrange & vocals are smooth but not forward, bass is authoritative but not boomy, highs are sweet & not tizzy. Soundstaging is appropriately deep with minimal inside-the-head affect. It is very addictive. Wes Phillip's 1999 (I think) Stereophile rave-review of the original was very accurate.
By the way, the Senn 650s are way more natural & better in every respect than the old 600s. (My cdp is an Accuphase DP75V. ICs are Kubala-Sosna Emotions. Power cables are TG 688s. I also use a BPT BP Junior power conditioner on the CD player. A great little headphone system, I believe.)

Check out the Cary CAD300-SEI. My Cary dealer got his Wednesday. 15 watts SET, with a selectable headphone jack on the front.
In my opinion:
$4500.00 - expensive!
The sound - better at low levels (driving speakers) than my Aleph 3, Aleph 30, Rocket 88, or any other amp I've ever heard. The bass is not as good as the Alephs, everywhere else the sound is better.
The current motif is black anodized faceplate, with Jaguar metallic blue paint, really nice!
Yes, I want one. Now I've got to sell something...

Go to headfi.com to get all the detailed scoop. There are a number of boutique manufacturers there.
Singlepower is one that has been very well received; has a number of models with all sorts of customization possibilities. I have an Audiovalve RKV MK2, which is quite punchy and wakes up the sometimes laid back Senns. Rudistor is another small shop from Europe which has some top notch product.