I'm not sure I'd make a blanket statement that the Kontras' readings of the Holmboe quartets are problematic, or not so good, or whatever derogatory adjective you want to use. Without other interpretations to compare them against, it is difficult to make any such judgment, and so I'll be interested to hear the Nightingale version. I do agree with Madiel that the audio quality on the Kontras' first CD (1, 3, and 4) is a bit weird, and needs a little tweaking of the equalizer settings - and surround sound (I think the recording might be out of phase - to sound really good. I'd say the same thing of the audio on #6 (but NOT 2 and 5, on the same CD). Those technical complaints aside, I'm generally pleased with the Kontras' musicality in these quartets, and they certainly sound to my ears like idiomatic Holmboe through and through.
BTW, something to keep in mind re: Holmboe's string quartets: there are NO "early" works among them. #1 comes between the 6th and 7th Symphonies and is remarkably mature. I listened to it yesterday (Kontra) for the first time in about a year, and I was enchanted by the second movement (especially) with its arresting, forte (or maybe even fortissimo) double-stopped passages and constant changes of texture and dynamics. It all sounds quasi-Bartokian, but you'd never mistake it for Bartok - Holmboe has very much his own distinctive voice by that point in his output.