Honestly, 10k is a fine output impedance for gear designed for 600, but is it different?
If the two devices on the right are in the 40k range, then yes. You'll be dropping the impedance a lot. Remember that parallel impedance is never higher than the smallest number.
In this case though if you had 2 x 40k sources you'd start wiht a 20k effective output that then gets cut to ~ 6.7k.
The formula for parallel impedance is 1 / ( ( 1/r1) + (1/r2) + (1/r...) )