The thing I don't like about the DP-01 line, is that it's A-in/B-in architecture assigned to 2-tracks. There's no front end mixer in this design. Sometimes setups demand front end mixing to the recorder, and so DP-01 and similar unit's owners get an outboard mixer, which solves the problem, but it's no longer a self-contained solution, (i.e., Portastudio). In these units, mixing is seen as something you only do when you play back the tracks. I think that's wrong. Designs like this generally get the thumbs down from me, 'cept if you're going ultra-low end like with the Porta 02.
The DP-01's moving design towards knob functions was good, but it's still menu driven at heart, and the knob section is hardly what I'd call a mixer. It's a great idea, but isn't fully implemented. It's a playback mixer only. I generally give a thumbs down to a menu driven system, anyway. YMMV.
On the other hand, (f/i), the Fostex FD-8 has an 8x2 analog mixer section that records to a 16/44.1k digital hard disc, up to 2-tracks simul (or 8-simul with ADAT I/O). With this design, you may do input-side mixing down to disc while recording, and output-side mixing at mixdown. It's an older but worthy design, & internal hard disc is optional. It's menu driven, but it's tolerable because the mixer section is fully implemented, (in that cheapie Fostex way).
Units that record only up to 2-tracks simultaneously are inherently limiting. Put that together with a straight A/B raw input, and what do you have? The DP-01 and the Porta 02.
When the DP-01 does flexible 8-input mixing to 8-simul recording,... call me.
Of course, the 688 has a fully implemented 8x8 recording mixer, with smart digital driven menu settings for patching and routing. You have a lot with the 688.