Recommendations please for good, affordable pre-amp for system?

I love the Thiels, although you're pairing speakers that could potentially be described as on the "lean and bright" side (at least that's how I recall hearing them), and an amp that definitely fits that description.  You might look to a preamp that could add some warmth, fullness, and flesh to the sound.  That's a tall order for under $500.

One such option is the Marsh P2000t (which I used to own many years ago).  Or you might consider a preamp from a company such as NAD or Parasound.  Or, if you're handy with a soldering iron, consider one of the Bottlehead (vacuum tube) preamp kits to put together.  

To be completely honest, though, I'm with jmcgrorgan2 in that I think you should sell the Bryston and use the proceeds plus that $500 to get a really good integrated that mates well with your speakers.  I truly think you'll be much happier in the long run at that price point.

Otherwise, if you can save another $500-1000, you'll be able to choose preamps in an entirely different league that will fill out your system much better.

Michael

@donzi, you're all over the map here.  My best recommendation would be to establish a relationship with a local dealer and borrow a variety of equipment so you can listen to the differences for yourself and best figure out what you tend to prefer sonically.  I think this advise will save you a tremendous amount of time and money in the future.

That said, I think the Thiels are excellent speakers, but difficult for all but the more high-powered tube amps to drive well.  Those tube amps won't be available at the price point that you'd be looking at, so that leaves solid-state amplification.

If I were to recommend an integrated, I'd think the Plinius 8000 or 9000 series would be a good choice, but consider also Classe, Electrocompaniet, NAD master-series, Yamaha AS-2000, or one of the older Musical Fidelity integrateds, just to list a few.

Ultimately, though, audio preference is a personal thing, and you will likely find your own preferences will evolve over time.  So the more equipment you're able to listen to, the most informed you will be both in terms of the differences (or lack thereof sometimes) of the various electronics, but also what you tend to like sonically.

Good luck, and enjoy the journey!

Michael