I did a test on this to comparing the following:
1. Using my Yamaha 01V96VCM to sum 5 stereo stems (10 channels) from Pro Tools 9 recorded back into Pro Tools (lightpipe routing) Channels panned hard L/R and faders at unity.
2. Summing in Pro Tools, but recording the output digitally routed through 2 channels of the 01V96, back into Pro Tools (lightpipe)
3. Bounce To Disk
Although the difference was almost negligible, I found that the fullest, warmest, detailed sound was the Pro Tools playback routed out and recorded back in. The 01V summed mix seemed a bit warmer, but less detailed. The bounce mix was most obviously different as it seemed to lack depth in the stereo image the other 2 had.
Small differences in digital summing, may have to do with the bit-rate and architecture of the processing: The Yamaha mix bus is 32 bit fixed point, whereas Pro Tools 9 is 32 bit floating point.
The source was an acoustic jazz trio of excellent New Orleans musicians
1935 Mason & Hamlin Model A Grand Piano
(2) Octava MK-12 & 1 TLM103
Mapex Drum Kit - Zildjian & Istanbul Cymbals
(2) Octava MK-12, AKG D-112, (5) SM57
(2) CAD E-100
All 13 signals through Focusrite ISA preamps & ISA A/D cards lightpipe into 01V96, then MOTU 2408.
1-2 Drum bus (8 tracks)
3-4 Bass bus (2 tracks)
5-6 Piano bus (3 tracks)
7-8 Reverb 1 bus
9-10 Reverb 2 bus