Would the HD621 do a similar thing as the 518 for 2-channel audio? An SPDIF input on a scaler, such as the VP50pro, could be used as input before the HD621. Or is it not recommended to enter the HDMI path with potentially more jitter to solve a jitter issue?

Holodeck 2: DSP5k2SE+7k2HCSE, DSP33, G68J, 621, DD15, OPPO-203, VW790, ST130G3-120", TSU-9400/RFX-9600, PS4, SB3, Buttkicker-XL (DIY JL 12W7)
In Kitchen: P200Z, T-amp, Equalizer, SB2