lot of misinformation here
You have an impedance mismatch between the amp and preamp.
this has nothing to do with input/output impedance matching
The Bryston puts out 15 volts, that’s a lot. Attenuating the signal (pad down) is an acceptable method to knock down the output signal.
15v of output on the Bryston (30 in balanced mode) and 29 db of gain in the Classe amps obviously is not a good match.
Maximum output level of the preamp is not the issue, the issue is gain
Get a better pre amp
This has nothing to do with how good a preamp is, whatever that means
It is called a system for a reason and your system has the same problem that many do. You have way too much gain if you can barely turn it up. Looking at the Bryston manual it says it is internally configured for either 16.5 or 22.5 dB so you could make sure you are on the lower setting. That may help but it won’t solve the problem as you will still have around 50 dB from pre + amp.
as others have suggested, you need less overall gain by either inserting attenuators or getting a different preamp that doesn’t have so much gain. I suggest a passive, which is basically what the suggested attenuators are , just that they aren’t variable. A power amp with lower gain is also a possibility but mid 20’s is pretty much standard so not a lot of options there.
I agree with the idea that it is rather silly to add +20 dB or so with a preamp and then take it away with a fixed -20 dB attenuator. Get a preamp that integrates into your system