From Stereophile "Bryston describes its SP2 multichannel preamplifier-processor ($4995) as consisting of a stereo analog preamp with a volume-controlled 5.1-channel analog pass-through plus a full-featured multichannel digital audio processor, and claims that none of those functions compromises any of the others. The analog preamp is fully equivalent in features and performance to their BP26 preamp. The digital processor includes all the latest Dolby Digital, DTS, and THX modes, and is based on Texas Instruments' powerful Aureus DSP chip, which can be updated via an S/PDIF input. The digital and analog sections have independent power supplies, and there are no video inputs or functions other than a control port for the optional, external SPV-1 video switcher"
The bp26 may be more refined as you stated but used prices on the sp2 and bp26 are seperated by over a thousand dollars .
Also per James post here -
http://www.audiocircle.com/index.php?topic=104614.0"If you are using the 1.7 for analog 2-channel only the upgrade is not necessary. It is only the digital board that gets the upgrade. The upgrade is around $1500 but check with Mike Pickett on that - mpickett@bryston.com.
Also check with Mike on the software but again if only using for analog 2 channel listening no need to be concerned."
So if that is correct (unless it changed down the road as I noted above which is possible), then there were no changes to the SP1.7 vs. SP2 on the analog side.
The SP1.7 manual indicates:
"Press the button once, and the unit goes in the Bypass 2ch mode. In this mode,
the LED will be GREEN. In Bypass 2ch mode all of the DSP circuitry, DA
and AD converters are bypassed, allowing a completely analog circuit path,
identical to the reference standard Bryston BP-25 pre-amplifier."
So they reference the BP-25.