Threshold input impedance ????

I do not know the exact number however I know that all Threshold series pre & amps were IGFET designs. Given this fact, the input impedance will be very high for both single-ended & XLR.
If your pre has a nominal output imp. of 600Ohms or something higher like 1 or 2Kohms, you should be just fine using the S450e, IMHO.
FWIW

Thanks for you reply. The reason behind my question is that I am planning on buying a pair of MIT balanced interconnects and they are input specific (Low/Medium/High) and I need to pick the right one. Here is how MIT devide them: low (5-50-kW), medium (40-100kW) or high (90-¥kW) .

Musicman159,

Looks like I spoke too soon! Disregard my prev. post. Sorry!

Here is a thread that might interest you:-
http://forum.audiogon.com/cgi-bin/fr.pl?aamps&1071852000&openfrom&1&4#1

The Input Impedance is 50K Ohms unbalanced and 600 Ohms balanced.

Lou

Some years ago I had a Threshold SA-4e. The input impedance is 50 K Ohms unbalanced and 600 Ohm balanced.
I had an excellent Magnan Signature XLR cable which by design has a very high impedance of 30 K Ohms. My local Threshold service simply removed one resistor per channel which changed the balanced input impedance to something like 50 K Ohms.
It was easy and without any problems because this resistor's role was simply to lower the input impedance to match a studio norm of 600 Ohms. Thus removing it did not affect the amplifiers basic design or performance.
My Magnan Signature worked excellent with the SA-4e after the modification.