Aragon, Plinius, or McCormack

I've not listened to any of the amps you mention, but based on my experieince with McCormack amps and what I've read about the McCormack DNA 0.5 I would no doubt go that route.

The DNA 0.5 is supposedly a legendary amp in it's stock form, and if you were to have McCormack perform a Revision A mod to it, it should become an entirely different beast still.

-IMO

Over the long haul, I might agree with Stehno, but point out a few things.

The Thiel 2.3 is a very resolving speaker. I can definitely see its attraction, BUT I would hesitate to feed it anything but the very best electronics. I suspect if you heard it and fell in love with it, you probably heard it with associated equipment much more advanced than the budget level you are looking to fill. (If not, all the better because you likely haven't heard it at its best.) To this end, I would advise a few things if you definitely don't want to change your speaker selection.

1. Don't skimp on the CDP/Source. Worry about this before your amp/preamp, or you will likely be very disappointed no matter what your other selections. The best low cost CDPs I could personally recommend are the Audio Note 2.1(?)or maybe a modified Jolida.

2. Your budget of $1k on an active preamp is also tight. Choose wisely or just go with the best integrated you can find. The Plinius you mentioned would probably be very nice, but I'm guessing that an LFD Mistral would work very well with the Thiel in a modest room and listening levels (check the Thiel's specs though). If you do go separates, maybe a used RLD-1(which is also upgradeable) or Rogue preamp would be a good option at that price level.

3. Amp wise. The DNA.5 will serve you for many more years than your other options if you go the separates route. As Stehno mentioned, it can be continuously upgraded to compete with just about anything out there at a fraction of the cost. If your long term goal is to have a SOTA system, this is the route to go. You will not believe what that old, but pretty decent DNA.5 can be transformed into. FWIW, Steve McCormack just developed a new modification -Carbon Matrix Wire Transformation - which I can vouch is well worth the $$$.

All that being said, SOTA CDPs, preamps and amp modifications don't come without a cost. If your budget for the entire system will be constant for quite awhile, I think you should abandon the separates route and get the best CDP and integrated amp you can find. Frankly, I think pairing the Thiel 2.3 with an Audio Note CDP and the LFD Mistral/Plinius integrated would yield a very nice system that would be very competitive with much more expensive ones.

Labtec makes some very good points. I will disagree with #1, however. For $1k +/-, you can get a BAT VK3i if you shop carefully. This mates very well with the DNA 0.5 and can even keep up with the SMc mods up through Rev B. Not until I got to A+ did I feel the need to upgrade my pre, at which point I went to a Supratek.