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

You'll regret selling that Bryston in favor of an integrated....  I have a DAC that can drive an amp and although it still sounds good, it can't drive my amp like my preamp can.  Be patient, maybe listen to a table radio a little longer and save a little more..... That amp and speakers were $$$ when new, they are worthy of a nice pre.   

Of the ones you noted, I would stick with the Bryston.

Many people like to overthink things and mix/match brands of preamps/amps.  Sometimes it works fine, but other times it's a disaster.  

It can be particularly risky with passives where I've heard plenty of combinations that SHOULD work on paper, but they don't.  Again, sometimes it works fine, but is it worth the risk???

Don't overthink things...You have an amp by Bryston that I assume you like and are keeping...why not just get the preamp from them which almost assuredly would be a good match?

The odds of you hitting a bigger home run with the mix/match route doesn't outweigh the risk of striking out, imo.

If you are looking to hit a home run or get a sizable upgrade, then I would suggest going with both ARC preamp/amps.  Otherwise, you should be just fine with Bryston in that price range.

@oddiofyl , you really need to listen to some good integrated amps before you condemn them. There are very nice units out there these days from Ayre, BAT, Pass, Hegel, Allnic, not to mention my VAC Sigma 160i that put a hurting on many separates these days.
That even address' @labtec post about running with the same brand preamp/amp.
I sold over $20K worth of separates when I heard the VAC 160i.
Sure, that is probably not a good match with the OP's Thiels, but I bet the Pass, Ayre, Hegel or BAT would drive them just fine.
Oh, and one less power cord and pair of interconnects to buy. ;^)

Donzi, I think you heard me correctly. You are in a situation where you have the potential for a great system, however currently short of funds. 
Don’t panic, Buy a used preamp that can be sold when you upgrade.  After all you can’t tell what you have without a complete system and the ability to listen comes 1st.

Your amp has an input  impedance such that it would match with most preamps, tube or solid state. 50K unbal, 30K bal.

I mated a tube preamp with a solid state amp. I like what this does for a digital system. Not saying this is what you need to do.  I would search the AG marketplace, purchase a pre that will serve you short time and when in a position to upgrade you can sell without losing many $ considering your use.

Take a look at the AG market place.

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