Are the 8 signals that you want to sum coming from the 8 DI's or from 8 mic amps that the DI's feed?
The 8 signals that you want to sum, do you mean 4 channels feeding left and 4 channels feeding right?
Combining signals passively means a loss. So, if you're combining the signals from the DI's then you'll need another 12dB from the pair of summing mic amps in addition to the loss that the DI has already incurred. Also, the resistance/impedance that the make-up mic amp is expecting to see is somewhere around what a typical mic impedance would be. Your 10K resistors will present too high a source impedance.
So, assuming you're summing straight from the DI's and we're talking about 4 stereo pairs... shooting from the hip:
I'd use an 800 ohm resistor for each polarity of the DI's balanced output. IOW, each DI presents roughly a 1K6 output impedance. With 4 channels, the total source presented to the make-up amp is about 400 ohms and we have about a 12dB loss through the combining network. Also the load looking backwards to each DI is in line with what it is expecting to see.
400 ohms might be a bit too high of a source for the make-up mic amp so shunting another 400R resistor across the final output will bring it down to 200 ohms. Right in line with what the mic amp wants to see.
However, this 400R shunt will result in another 6dB loss - for a total network loss of 18dB. Use your judgement as too whether this is an OK additional amount of gain to have to make up.
Juggle around with something like that and you should be fine.
And if I misunderstood what you're doing, just ignore this
