I too have found the Krell 400xi to sound much better in balanced mode vs. single-ended. I also found this Amplifier to sound “great” when paired with the Krell SACD Standard (Stereophile A+ rated), especially in balanced mode. The + 6dB gain is quite noticeable and appreciated (less work for the Amp. to do), plus on SACDs the Standard adds + 5.5 dB via Filter 3, for a whopping 11.5 dB output gain! Like certain components from other manufacturers, the Krell 400xi & SACD Standard are sonically, functionally and aesthetically (look awesome together in Black) made for each other. As per Krell, these two particular components were co-developed side-by-side (Krell currently markets them as the “Executive System), and have received excellent reviews as a system. For some Buyers of the 400xi, the Standard might be a financial stretch (@ $4,500) to purchase, but it is well worth the investment, I feel that this Player squeezes-out every once of Class A performance from this Amp. All of the Tweeter/Sound Advice stores that I've been in as of late have had this combo. set-up, usually paired with ML, Krell Res. 2 or Focal 1027 Be Speakers, and the sales-reps tell me that they sell very well together.
Incidentally, I recently purchased the 400xi/Standard/1027 Be combination, set-it-up in a fairly large 25’x20’ room and couldn't be happier. For $15K, this system can hang with the very best systems that I have heard. Save-up your money, and then buy the Standard to pair with the 400xi, you’ll have no regrets, besides not having done it sooner.