BPT 3.5 Signature vs. 3.5 Sig PLUS

I have owned both units as well. My findings are similar to Owl's. The BPT 3.5 Signature was excellent, however a bit closed in on top, or perhaps a bit dark overall. The 3.5 Signature PLUS not only seems more neutral in overall character, there is more extension in both treble and bass, and the noise floor is even quieter - no joke. I have my amps plugged into the unit, and Chris Hoff at BPT gave me the tip to try the amps plugged into both the "amp" plugs as well as the duplex next to it, as one has different sonic characteristics than the other, and system matching will determine which works best...

I personally feel the upgrade between the 3.5 Sig to the 3.5 Sig Plus was well worth the cost difference.

Anytime Grant. Yes, in my experience the Sig Plus seems to posess more natural treble extension (shimmer & decay on a cymbal hit, etc.) while overall being more neutral and not as "slightly dark" overall as the Signature model was in my experience. There is also improvements in lowered noise floor, extension in bass, dynamics, and to me the midrange seems to have more "purity" per se... perhaps the lack of "slightly darker" tonal balance that I thought the Sig. model had. As I mentioned above, Chris @ BPT will tell you if you call him, to try the amp plugged both into the amp duplex as well as the duplex next to it, as they both have different sonic characteristics.