XLR or single ended cables and why?

Simple answer: With your Krell gear, you should definitely try out a balanced connection (and this is regardless of how long your cable run is). You don't need to commit first to purchasing the XLR interconnects if you can borrow a pair from a friend or a store demo for audition. Of course it would be ideal if you could either get ahold of a pair that corresponds to your usual reference single-ended IC's, or else wrangle pairs of the same audition model in both XLR and RCA versions so that the comparison isn't confounded by uncontrolled variables. Then just listen, not for noise per se, but for probably somewhat subtle (which is not to say unimportant) musical differences, especially concerning transient, dynamic, spatial, tone color saturation, and transparency/resolution qualities. Remember that because a balanced connection will usually yield a higher volume than single-ended (typically about 6dB hotter), for valid results always compensate with the volume control to equalize levels when performing A/B comparisons.

The question here isn't whether Andrewdoan should buy components with balanced circuits, since he already owns them. Anyway, in home audio this choice is rarely about reducing overt noise levels. For a combination of reasons -- running true-balanced is capable of reducing not only induced noise, but also self-generated noise, and in some cases distortion too -- the pertinent point is that the sound of the music might be improved by using balanced connections when running balanced gear, even if you never hear the result as being 'noise-reduction' per se. One of those reasons is that internally-balanced gear typically subjects unbalanced input signals to an additional circuit stage in order to convert them to balanced for processing, and this extra input stage is bypassed when using a balanced connection. (Conversely, gear that isn't internally balanced, but merely has XLR jacks wired in series with its RCA jacks for the sake of convenience, will often sound better with single-ended connections, which dispense with the extra jacks, wiring and solder joints at this small-signal juncture). So the answer to the question at the top is that there is no one right answer, it depends on the gear -- and with gear that was designed to make full use of a balanced connection, it makes sense to try one.

Andrew: The SPL is determined by the volume control setting. (We needn't go into the subject of dB's, watts and ohms, but suffice to say that it's not as you suppose.) Anyway, don't worry about any of this -- everything will behave as it did, except that you may find your preferred volume setting to be a couple clicks lower.