What is the best preamp for Carver monoblock tube amps?

IMO, get the Carver, expect your pre-amp to sound great. if not, then search

Which model of Carver? Do you know the input impedance?

Other than being able to drive the input impedance, the correct answer is along the lines of 'the preamp that you like the best'.

Get the order placed for the Carver amps, may take a few weeks if not in stock on the shelf.

 Get the preamp a tune up is all.
amazing preamp!

get an Octave 10 Audio pre amp.  That's what I have connect to my 350s.  Hand wired, great tubes and made in the US.  I love mine, if you want to spend 3xs the money for a name (that's not the quality and handcrafted). your friends will know that's up to you.  My S# is 13 on the 10 btw.
bestMark

atma......
Under 1-ohm capability  Input Impedance:
100 K ohms (RCA)
Source Impedance:
1.7 ohms not sure the spec you are looking for for the Carver 350s  I assume 100K but....
BestMark