Hi Alex,
Its not a simple yes or no answer I think ,lets say for the sake of simplicity your using a 1:1 transformer on the output ,with the matching impedence of 10 kohm you most likely will get a small bump in the high end frequency response,at the same time your transformer will be more or less unloaded ,which will reduce the amount of power the transformer has to pass,and reduce the amount of transformer core distortion.Lets say you use a 1:4 transformer ,you will end up with about a 12db gain from the transformer itself , this will mean you might be wasting headroom and not driving the circuit fully .Depends alot on the input level your convertor needs to hit 0db fs digital.So if your preamp is capable of say +24 db at the output ,you add a 1:4 step up transformer ,now when your circuit is driven to full level the output from the transformer will be up around +36 db which is way too much level for almost anything.If you have a look at my previous post ,the transformer Im using has a step down ratio of 4:1,so Im loosing 12db of headroom.Although 600 ohm inputs are rare these days its generally a good thing to have a preamp capable of driving low z.What might be a good plan is to terminate your transformer with a variable resistance ,say a 20kohm pot, inject a squarewave of a few khz,monitor the output waveform ,and adjust the pot to get the best squarewave output without overshoot at the leading edge of the waveform,once youve found the sweet spot ,measure the pot value,and put this value of resistor across the outputs of your preamp .Heres a usefull piece of software that might help,
http://www.ymec.com/products/dssf3e/
It has a signal generator and a scope and works with almost any soundcard,
best of luck,