Dimension Pro Bass note limits

Modify the sfz files, the key setting in the MIDI DPro track and base octave display in Preferences to suit your needs. Did you check the fundamental frequency of the notes played? Using the defaults, the first Bass program 03 - Real Basses > Dull Fingered 1.prog shows E3 playing at ~41Hz which is open E using standard tuning on a 4 string electric bass. The program also plays down to ~29Hz which by default displays B2b.

In the analog world E1 is 41Hz. You were asking about DPro in SONAR. Remember sfz works with MIDI note numbers and maps those numbers to samples. How MIDI note numbers relate to pitches displayed in SONAR is set by "Base Octave for Pitches" in Preferences > Customization > Display. You could modify the sfz files, adjust the Key+ slider in the track inspector to make programs appear to play in a certain range, or change the display. I believe the Key+ slider is less work than messing with sfz files and more targeted than the global change in Preferences. Granted changing the sfz files is a one time event.

Many bass synths transpose the key mapping to sound an octave lower than normal on any given MIDI note number. This faciliates playing bass on compact keyboard controllers with a limited range. Most, if not all, Dim Pro bass patches do this. In addition, there is no universal standard for octave numbering.  SONAR labels Middle C (MIDI Note Number 60), C5. Some software/hardware vendors label it C3 or C4. SONAR's convention eliminates the need to use negative octave numbers to cover the full MIDI note number range.
 
The important thing is that the synth can play a low E at 41Hz, regardless of what MIDI note number or DAW octave that sample is mapped to.
 
As Steve indicated, if you want the bass part to display in the 'normal' octave in the PRV or Staff view, the easy way to do that is to enter +12 in the Key+ (Key Offset) setting in the MIDI/Instrument track for the bass.