Compare Bryston BP173 with Bryston BR20

You're welcome. One other difference is that the BR-17^3 has HT bypass, while the BR-20 does not (although it may be added later).

Thank you for your response. I was just curious as to what differences that I might hear listening side by side.
 I thought it interesting that Bryston chose not to include the Salomie circuitry in the BR20.

In my opinion the BR-20 is a clear improvement across the board when compared to the BP-17^3 (we have both models here). For one thing, the noise floor is significantly lower. Overall clarity is better as well. There’s a reason Bryston calls the BR-20 their best preamp ever.

There are some features on the BP-17^3 that you might still want, such as single-ended output jacks (the BR-20 can use single-ended with specifically wired adapters), more analog inputs, or RS232 control. The BR-20 trades several analog inputs for digital inputs.