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

SS amp does not require a SS preamp. In fact many folks like to couple a tube preamp with a SS amp. You just have mind the impedance mismatches. It would appear that you would be fine with a preamp with an output impedance under 1K.

That said, looking for a >$500 preamp to mate with that much more expensive amp would seem a bit strange to me. I think your only option would be to go with some passive attenuators.

Other options would include, look for a digital source with volume control to replace your current one, or look for a integrated amp to replace your Bryston amp.

I do agree with mesch, I find the preamp the heart of the system. For me, finding the right preamp can be more difficult than finding the right speakers.

donzi, you are correct. I had the > turned the wrong way, should have been <$500.

There is virtually no active preamp on the site right now that would do justice for the amplifier and the speakers that you have.
There is a nice passive unit, a Sonic Euphoria PLC passive line stage for sale on Audio Asylum for $625, maybe $500 gets it. As far as fitting in with the quality of your amp and speakers, that’s probably about the best you can do.

I would still consider selling the Bryston and going the integrated amp route, or using a source with a built in volume control.

@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 , the 300B is tube usually used in SET amplifiers. The only preamp that I can think of that uses the 300B is the Manley Neo-Classic 300B.

I agree with @sufentanil that the Bryston/Thiel combo would be too bright for my tastes. However, I do understand why they make so many different flavors. Different strokes for different folks. Some people love extreme definition.

I have a friend who loves his Thiel 3.6’s. He has found the best combination to be a tube preamp (CJ), Classe amp, and Kubala Sosna cables. All of his electronics and cables have a reputation for being warm sounding, which is why they probably mate so well with his Thiels.

As for integrated amps, I don’t know that a tube integrated amp will drive these speakers properly.
I think the Thiels crave SS power. A BAT integrated may give you the taste of tubes though, with a SS output stage (150 wpc). Pass Labs INT-150 is also very popular, though no tubes are used.

Also, how large your room is and how loud you like to listen will determine how much power you need. Will the 125 wpc of an Ayre AX-5 be enough? Or do you need the 250 wpc of a Hegel H360, or maybe even the 400 wpc of the CODA CSiB or Jeff Rowland Design Group Continuum S2? Only you can make that decision.

Happy hunting,
John

Congrats! Enjoy your audio/video processor!

@jond , Donzi bought the Theta Digital Casablanca 6-Channel Preamp Processor.