No clear agreement on these things. Amplifier input impedence on the Alephs is around 11Kohms I think. Rule of thumb is to have this value be greater than or equal to 10 times the output impedence value of the pre-amp, though some people say you want a lot more than 10x. Many SS preamps have output impedences below 100 ohms, but some tube pre-amps are closer to 2K ohms.
A mismatch is not the end of the world: you may lose a little bass and some other things, but it's not always a non-starter. Just be mindful of it.
Hopefully someone smarter and more technical than I will weigh in. If you do a search in the forums for "impedence" or "mismatch," you will find more info.
