It's been said a number of times - Spectral amps are current driven, and very few DACs or other preamps can properly drive them. Spectral eased the language in some recent amps, like the 400RS, but tightened it again in the 300RS. That alone creates user confusion, and understandably so. Other functions provided by their preamps include DC and oscillation protection, and I have triggered the DC-offset protection circuit a number of times with my Pass XP-25 myself; apparently, it is not all that stable in that department.
Tasos is correct but there's more. Spectral has flip-flopped on this issue more than Mitt Romney. Although my 400 owners manual specifically says the only requirement is to use a low output impedance pre-amp, they have now instructed their dealers not to sell a Spectral amp of any kind to someone not using a Spectral pre-amp. This is at least the 3rd or 4th flip- flop in over 20 years. Wow. It's more confusing than "fat Oprah?" thin Oprah?" Make up your mind already Spectral and stick to it!