Has anyone used the aesthetix calypso preamp?

Well, Bhouser, my point is, I have not seen anything "Official" from Asthetix on this subject. As I stated, you would think mention of this would be in the owners manual if it was a problem. I plan on calling them tomorow and asking about this at which time I will post the information.
There are a lot of others factors that can cause reduced dynamics, etc. besides impedance mismatches between components such as input sensitivity of the amp versus output of the pre associated with input sources outputs. Normally, an impedance mismatch will cause a roll off in the highs and not so much affect dynamics. This roll off wouldn't be too bad but would exist. Most tube preamps like to see at least 20k.
I'm not trying to dismiss your concerns but the Calypso is a WELL designed preamp in every technical sense. I just can't believe a company manufacturing tube preamps would require a 50k impedance which seems awfully high.
On another note, I can't believe an output coupling transformer would increase dynamics. How exactly is it doing this because it is not all about impedance you know. I would think this would limit bandwidth and create other problems.

Bigtee-

Very good points. Please report back with us when you talk to Aesthetix.

I forgot to mention an amp's input sensitivity above - which is yet another important factor in the BLACK ART of system matching. Things like this never get any air-time when people openly recommend preamps they own.

But, as I said above, minor frequency/extension rolloff COULD be a potential problem with an impedance mismatch, so again, I agree with you.

As to the Calypso specifically, I researched it heavily and really liked what I saw, but then I read those few comments about poor dynamics or rolloff. Finally, the 50k manufacturer recommendation really pooched the deal and I went no further.

From the research that I have done, transformer-coupled tube output designs are a unique breed of preamp circuit, that has proven to generally combine the dynamics of solid-state pre's without sacrificing the mid-range magic and delicacy of tubes (when they are executed well). The VAC Renaissance series and some of the current Supratek and First Sound models (among others) use this basic design to help eliminate system/ground noise, to drive long interconnect runs and to match the impedance curves of virtually any power amp.

As to HOW it is specifically designed to improve these things and why it doesn't seem to cause other problems - I couldn't tell you. But I am sure that Kevin Hayes at VAC or Emmanuel Go of 1st Sound would be happy to.

I DID compare my VAC Ren to 4 well-regarded preamps - a tube hybrid and 3 solid-state pre's, all connected to my 'low impedance' 20kohm Pass X-250 amp. The VAC was clearly more dynamic and punchy then all 4, while it provided the 3-dimensional midrange that the SS pre's could only dream about.

All I can tell you is, I have NO problems with dynamics, extension or impedance-matching in my system... so it MUST work!
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Sounds like it does and has made you happy in the process which is all you can ask for. I am not that familiar with the First Sound (with only a short listen) nor the VAC for that matter. I do have a friend who used the First Sound for a little while. I have stayed more with main stream products that I can readily listen to and/or borrow(which is getting pretty dang slim.)
I always try to make sure everything is compatible that I purchase and it does require homework to succeed. The Ayre is an incredible match with the Calypso(in fact, the guys at Ayre even recommended it.)I'm sure they are friends with Jim White of Aesthetix since they (along with Charles Hansen) have joined forces at Theta for a few products in the past. All of these guys believe in using zero feedback designs. So, all of this falls in line with my belief of time aligned, phase coherent speakers and components that are zero feedback that offer good time domain performance.
I have listened to the ARC Reference Mk.II, the BAT 51se and a few others in my system. The Calypso easily held its own for less money(WAY LESS!) So, That pretty much sold me own it.

Has anyone used a VK 75se with a Calypso?

As a quick note. I have been trying the Calypso pre amp for a few months now. First off it has surpassed my Placette Active line stage in all ways. I have it hooked up to Plinius SA-102 amps at 47ohm input and have had no issues. I have found the stock Sovtek tubes to lack in dynamics, bass and extension. The Mullard anf Telefunken (sp) are great!!!!!!!