I know many EE DAC users won't be swapping op-amps, but if you think you might, I'd say do it now before you get all tube-rolling crazy with it.
The op-amp swaps, IME, play a greater role in the sound of the unit than tube swaps.
If you do not swap the op-amp in the I/V position that feeds the tube output, you will be limiting the performance of the DAC substantially, regardless of how many $$$ tubes you roll.
And if you swap both pairs of op-amps so that you have the SS output sounding nice, you just might care about what tube you have in there

.
IMHO, IME etc. etc.
-Mike