Krell does not use coupling caps at the input which can result in problems if DC is leaked in from the preamp (both for the amp and speakers). The upside is better transient response as the signal is not impeded a constant charging and discharging capacitor.
Even though the amp will have some sort of DC protection, it may not be enough to overcome a tube preamp DC spike per lynn_olson’s post above. You should contact that tech again and ask if the amplifier has coupling caps that can be activated in order to use a tube amp.