Trying to match impedance

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

Thanks for your help Mitch2.
The 1.2K ohm input impedance in balanced mode is correct according to the manual of the power amp.

Are you sure that 1.2K number is right? I'm going to assume that the number might be 12K ohm. This is still rather low for an input impedence.

I would probably steer clear of preamps that have output impedences greater than about 500 ohm unless it too offers a balanced output, in which case you can stand considerably higher. Most solid state preamps aren't going to present a problem, while valve preamps tend to have higher output impedences...though, not universally in either situation.

Where you might start having compatability problems would be with a valve preamp with an output impedence greater than 1k ohm. Generally, you want an input impedence of at least 10X that of the output impedence of the source feeding it.

To answer your last question, you shouldn't damage anything, but you will have a very pronounced roll-off of the frequency extremes and your speakers will sound a whole lot like their impedence plot. Where the speakers dip in impedence, your amp will struggle to provide current and those frequencies will be attenuated.

I suppose you could damage something in tube gear if you could stand to listen to the combination for very long.

Thanks for the info Montyx.
The input inpedances are correct, I have the manual in front of me.
The power amps I have are Threshold T-100 and T-200. The T-100 has the 1.2 K ohm input impedance in balanced mode and the T-200 has 2K while both have 50K unbalanced. T series Threshold amps were known for having extremely low input impedance in balanced mode.
Anyway, thanks guys, I see the picture.