SB Z or SB Omni should be practically the same at optical output, so I'd say whichever is cheaper.
The DACs built into those aren't bad, so I'd disagree with Rudy and say connecting the Vali to the headphone outs should be fine/better than the Mixamp setup you already use. You could always start SB>Vali>Q701 to start off with and buy an optical DAC later if you want, see if you can hear a difference. The DAC in the Optical Modi (1st gen) and Omni sound extremely similar through headphones.
The X7 basically serves as Sound Processor, DAC, and Amp, so you wouldn't need the Vali at all in that setup (unless you wanted to for flavor). That said, the X7 also has a ton of inputs, so you could feasibly use it instead of the Mixamp with your console too. You choose! It was on Massdrop for a week at $300, not sure if the drop ended yesterday...
Yea I think I'll start of with the sound blaster z > vali > q701 and see how I go.
I'm selling my ps4 plus mixamp so wouldn't get the use of x7 on PC and console.