what is my system's weakest link?

I have a different take on this. The "weakest link" is the Project tt. The biggest "problem" is the Thiel/Brio combo. The Brio is a nice integrated and while you mention wanting a new pre, what will you do about a power amp? The C28 is a good vintage pre, but do you really want to open up that can of worms (restoration/reliability of pre, new power amp)? If it were me and the goal were the best possible sound for +$1000 or so, I would build a new system around the Brio which is a solid integrated, get friendlier speakers (I like the Vandersteen suggestion) which you should be able to buy with what you get for the sale of the Thiels plus a little more, and GET A BETTER FRONT END; something like this (no affiliation):

http://app.audiogon.com/listings/turntables-vpi-traveler-v1-2013-11-06-analog-90039

or even better:

http://app.audiogon.com/listings/turntables-exquisite-well-tempered-lab-record-player-optional-dust-cover-with-marigo-audio-lab-p-2013-11-02-analog-94925-corte-madera-ca

I think you will have infinitely more satisfying sound this way. Good luck.

Moreytalmor, this is a perfect example of why it's important to give as much detailed and concise information as possible when asking for opinions on this forum. First of all, remember that these are just opinions (ours) and it's yours that count to the most. However, without all the facts we are forced to read between the lines. For example: THE ONLY comment in your post re the kind of sound you like was the reference to the Mc preamp as being "warm". OK, I figured, the guy wants a warmer sound. Reinforcing that assumption was what I know about the sound of the Thiels; they are not what I would call warm. Now we learn that not only do you think they sound great, but you like the way they look and seem to be reluctant to give them up. That's great, and nothing wrong with liking the way gear looks. OK, if they sound great then that tells me that the amp/speaker synergy is probably not too bad for your listening habits and tastes (notice I am still reading between the lines).

So, what areas exactly are you feeling the sound of your system is deficient on? What are you looking for? More fullness (warmth), more detail, bigger soundstage, more volume? Or are you looking for simply an overall, general improvement in sound from "great" to really great. Also, does your budget of $1300 include the proceeds from the sale of whatever it is you end up selling? What cartridge do you have on your tt?

Moreytalmor, my suggestion to you is to go slowly. You seem to have a
pretty good idea of the type sound you are looking for. This is all a learning
process and it is most educational to change out one component at a time.
You seem to want to keep the Thiels; great, that simplifies matters. While I
think you are a prime candidate for tube electronics, I would get a better
turntable/cartridge next and see how far that takes you to your goal. A
better turntable will give you more of the musical involvement that you are
craving. In fact, I might start by upgrading the cartridge first to one that is
really good and you can install on the better turntable that you will buy later
after you get a handle on your new sound. Don't sell the old cartridge; you
will probably want to sell the Project with the old cartridge. Don't be afraid
to spend what may seem to be a disproportionate amount of money on a
cartridge; rmember any information that you lose at that stage of things you
will never be able to get back no matter how good the gear upstream. Start
by reading this:

eanlg&1200430667&&&/Who-needs-a-MM-cartridge-type-when-we-
ha>http://forum.audiogon.com/cgi-bin/fr.pl?eanlg&1200430667&&&/Who-
needs-a-MM-cartridge-type-when-we-ha