The basic rule of thumb is to have input impedance of the amp be at least 10x the output impedance of the preamp. However, many consider this ratio too low and would be more comfortable with numbers of 20x or better. Also, preamp mfgs sometimes do not disclose that the output impedance can sometimes vary significantly with frequency, although the larger variations are more common to tube preamps. If your ratio is low, such variations can result in frequency roll off, most common in the bass causing the system to sound thin and/or bright. BTW, your reported input impedance in balanced mode looks goofy compared to the 50k value in single-ended mode. Maybe double check that. Also, you can search here and at AA where this topic has been covered more than a few times. Here is one thread;
http://forum.audiogon.com/cgi-bin/fr.pl?htech&1162815631&openusid&zzHodu&4&5#Hodu
http://forum.audiogon.com/cgi-bin/fr.pl?htech&1162815631&openusid&zzHodu&4&5#Hodu