I moved up from an Onkyo 805 to a Onkyo Pro PR-SC885P (same as Integra 9.8) processor, using the same external amps with both. The processor does sound better than the AVR. It has better D/A chips (Burr Brown), a toroidal transformer, separate analog and digital power supplies, VLSC circuitry, and other improvements in componentry. The upgrade will not make as much difference as when you added the external amps to the 705.
What you also get with the pre/processor over the AVR is another HDMI input and output, the option to use Audyssey Pro calibration, XLR input/output connections, HD Radio, improved second/third zone capabilities, and the HQV Reon video chip which is vastly superior to the video processor in the 705.
If these additional features are not useful to you, it would probably not be worth the cost difference to upgrade based on the improvement in sound quality alone. I thought that it was but YMMV.
What you also get with the pre/processor over the AVR is another HDMI input and output, the option to use Audyssey Pro calibration, XLR input/output connections, HD Radio, improved second/third zone capabilities, and the HQV Reon video chip which is vastly superior to the video processor in the 705.
If these additional features are not useful to you, it would probably not be worth the cost difference to upgrade based on the improvement in sound quality alone. I thought that it was but YMMV.