With the notice that you should be sleeping at this time and that the danger exists you will never get some sleep any more for this night :
I thought you dared to get rid of the pre-amp long ago my now ?
Using the preamp does not make sense in this realm (of the BNC cable) although it should be able to improve just the same. But why go through the effort if you can do half of it anyway, the other half being destructive behind your back ? Thus :
a. Using the preamp is for the worse (and if too much digital attenuation is needed get another power amp
b. Output impedance of the preamp will be 600 Ohm or so, and thus this (75 Ohm) does not apply there;
c. Meanwhile nothing really matches between preamp and poweramp, but this is another (new) story (but read : what prevents the reflections in that connection ?).
I tend to say : Do it all (meaning ditch the preamp to begin with) or do nothing. But this is not really different from me "forbidding" to use a preamp. Between quotes of course, but you know ...
Sweet dreams ! (ahum)