Would you borrow the Pass amp from a local dealer? Try a Pass X series pre-amp first. They are truly balanced - like the X 350. I think the KAV-250a is truly balanced?! You might want to try an Adcom GFP750 pre-amp also. It has single ended & truly balanced inputs & outputs, & was designed by Nelson Pass, who founded Pass Labs. I use the GFP750 with my Rowland Model 2 in balanced mode & it works quite well.
I've seen the NAD pre amp at a local dealer, I believe it is "psuedo balanced" which means although it may have "balanced" inputs or outputs, they are coverted inside the pre-amp to single ended anyway. Check the input & output impedance numbers. The Adcom input impedance measures 94k ohms balanced, 47k ohms unbalanced. Output impedance 1200 ohms balanced, 600 ohms unbalanced. When the balanced value is double the unbalanced value, the component is "fully balanced" or "truly balanced".
I also own a Pass Aleph 3 amp, it is single ended only, when I use it I run the variable out from my SONY XA7ES directly into it.
If I were you I'd give the Krell another chance with a better pre-amp.
I've seen the NAD pre amp at a local dealer, I believe it is "psuedo balanced" which means although it may have "balanced" inputs or outputs, they are coverted inside the pre-amp to single ended anyway. Check the input & output impedance numbers. The Adcom input impedance measures 94k ohms balanced, 47k ohms unbalanced. Output impedance 1200 ohms balanced, 600 ohms unbalanced. When the balanced value is double the unbalanced value, the component is "fully balanced" or "truly balanced".
I also own a Pass Aleph 3 amp, it is single ended only, when I use it I run the variable out from my SONY XA7ES directly into it.
If I were you I'd give the Krell another chance with a better pre-amp.